[tools] Simplify class_ptr rewriter to not create new instructions when not needed.

[rolfbjarne]

We can simply change operands of existing instructions instead.

Fixes these test failures:

MonoTouchFixtures.AppKit.NSGridViewTest
	[FAIL] CreateWithNSViewArrayOfArrayCheckDifferentArrayLength : System.InvalidProgramException : InvalidProgram_Default
		   at AppKit.NSGridView.Create(NSView[][])
		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithNSViewArrayOfArrayCheckDifferentArrayLength()
		   at System.RuntimeMethodHandle.InvokeMethod(Object, Void**, Signature, Boolean)
		   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
		   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object, BindingFlags)
	[FAIL] CreateWithNSViewArrayOfArrayCheckNSTextView : System.InvalidProgramException : InvalidProgram_Default
		   at AppKit.NSGridView.Create(NSView[][])
		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithNSViewArrayOfArrayCheckNSTextView()
		   at System.RuntimeMethodHandle.InvokeMethod(Object, Void**, Signature, Boolean)
		   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
		   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object, BindingFlags)
	[FAIL] CreateWithNSViewArrayOfArrayCheckWithNull :   Broken Array #3
  Expected: <System.ArgumentNullException>
  But was:  <System.InvalidProgramException: InvalidProgram_Default
   at AppKit.NSGridView.Create(NSView[][])
   at MonoTouchFixtures.AppKit.NSGridViewTest.<>c__DisplayClass6_0.<CreateWithNSViewArrayOfArrayCheckWithNull>b__0()
   at NUnit.Framework.Assert.Throws(IResolveConstraint, TestDelegate, String, Object[] )>

		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithNSViewArrayOfArrayCheckWithNull()
	[FAIL] CreateWithNSViewArrayOfArrayCheckWithNullArray :   Broken Array #2
  Expected: <System.ArgumentNullException>
  But was:  <System.InvalidProgramException: InvalidProgram_Default
   at AppKit.NSGridView.Create(NSView[][])
   at MonoTouchFixtures.AppKit.NSGridViewTest.<>c__DisplayClass5_0.<CreateWithNSViewArrayOfArrayCheckWithNullArray>b__0()
   at NUnit.Framework.Assert.Throws(IResolveConstraint, TestDelegate, String, Object[] )>

		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithNSViewArrayOfArrayCheckWithNullArray()
	[FAIL] CreateWithNSViewArrayOfArrayCheckWithNullNSView :   Broken Array #1
  Expected: <System.ArgumentNullException>
  But was:  <System.InvalidProgramException: InvalidProgram_Default
   at AppKit.NSGridView.Create(NSView[][])
   at MonoTouchFixtures.AppKit.NSGridViewTest.<>c__DisplayClass4_0.<CreateWithNSViewArrayOfArrayCheckWithNullNSView>b__0()
   at NUnit.Framework.Assert.Throws(IResolveConstraint, TestDelegate, String, Object[] )>

		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithNSViewArrayOfArrayCheckWithNullNSView()
	[FAIL] CreateWithTwoDimensionalNSViewArrayCheckDifferentDimensionSize : System.InvalidProgramException : InvalidProgram_Default
		   at AppKit.NSGridView.Create(NSView[,])
		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithTwoDimensionalNSViewArrayCheckDifferentDimensionSize()
		   at System.RuntimeMethodHandle.InvokeMethod(Object, Void**, Signature, Boolean)
		   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
		   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object, BindingFlags)
	[FAIL] CreateWithTwoDimensionalNSViewArrayCheckWithNull :   Broken Array #4
  Expected: <System.ArgumentNullException>
  But was:  <System.InvalidProgramException: InvalidProgram_Default
   at AppKit.NSGridView.Create(NSView[,])
   at MonoTouchFixtures.AppKit.NSGridViewTest.<>c__DisplayClass7_0.<CreateWithTwoDimensionalNSViewArrayCheckWithNull>b__0()
   at NUnit.Framework.Assert.Throws(IResolveConstraint, TestDelegate, String, Object[] )>

		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithTwoDimensionalNSViewArrayCheckWithNull()
	[FAIL] CreateWithTwoDimensionalNSViewArrayNSTextView : System.InvalidProgramException : InvalidProgram_Default
		   at AppKit.NSGridView.Create(NSView[,])
		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithTwoDimensionalNSViewArrayNSTextView()
		   at System.RuntimeMethodHandle.InvokeMethod(Object, Void**, Signature, Boolean)
		   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
		   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object, BindingFlags)
	[FAIL] CreateWithTwoDimensionalNSViewArrayWithNullCell :   Broken Array #5
  Expected: <System.ArgumentNullException>
  But was:  <System.InvalidProgramException: InvalidProgram_Default
   at AppKit.NSGridView.Create(NSView[,])
   at MonoTouchFixtures.AppKit.NSGridViewTest.<>c__DisplayClass8_0.<CreateWithTwoDimensionalNSViewArrayWithNullCell>b__0()
   at NUnit.Framework.Assert.Throws(IResolveConstraint, TestDelegate, String, Object[] )>

		   at MonoTouchFixtures.AppKit.NSGridViewTest.CreateWithTwoDimensionalNSViewArrayWithNullCell()

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✅ API diff for current PR / commit

.NET ( No breaking changes )

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

❗ API diff vs stable (Breaking changes)

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• iOS: vsdrops gist ( ❗ Breaking changes ❗ )
• tvOS: vsdrops gist ( ❗ Breaking changes ❗ )
• MacCatalyst: vsdrops gist ( ❗ Breaking changes ❗ )
• macOS: vsdrops gist ( ❗ Breaking changes ❗ )

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🎉 All 115 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 8 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

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