Set UnusedValue correctly for the operands of ARR_ADDR/BOX

src/coreclr/jit/rationalize.cpp

@@ -556,13 +556,6 @@ Compiler::fgWalkResult Rationalizer::RewriteNode(GenTree** useEdge, Compiler::Ge
             RewriteAssignment(use);
             break;
 
-        case GT_BOX:
-        case GT_ARR_ADDR:
-            // BOX/ARR_ADDR at this level are just NOPs.
-            use.ReplaceWith(node->gtGetOp1());
-            BlockRange().Remove(node);
-            break;
-
         case GT_ADDR:
             RewriteAddress(use);
             break;
@@ -594,9 +587,11 @@ Compiler::fgWalkResult Rationalizer::RewriteNode(GenTree** useEdge, Compiler::Ge
             break;
 
         case GT_NOP:
-            // fgMorph sometimes inserts NOP nodes between defs and uses
-            // supposedly 'to prevent constant folding'. In this case, remove the
-            // NOP.
+        case GT_BOX:
+        case GT_ARR_ADDR:
+            // "optNarrowTree" sometimes inserts NOP nodes between defs and uses.
+            // In this case, remove the NOP. BOX/ARR_ADDR are such "passthrough"
+            // nodes by design, and at this point we no longer need them.
             if (node->gtGetOp1() != nullptr)
             {
                 use.ReplaceWith(node->gtGetOp1());

src/tests/JIT/Regression/JitBlue/Runtime_70466/Runtime_70466.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.CompilerServices;
+
+public class Runtime_70466
+{
+    public static int Main()
+    {
+        Problem(1, 0);
+        return 100;
+    }
+
+    // This method is carefully crafted such that we end up with an unused ARR_ADDR node by rationalization
+    // time, of which the child operand will need to be explicitly marked "unused value" for LIR purposes.
+    //
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Problem(byte b, int idx)
+    {
+        var a = new byte[] { b };
+        var a1 = new byte[] { b, b };
+        var a2 = new byte[] { b, b, b };
+
+        if (idx == 0)
+        {
+            if (a[idx] == 1)
+            {
+                Use(1);
+            }
+            JitUse(a1[idx] + a2[idx]);
+        }
+    }
+
+    public static void Use<T>(T arg) { }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void JitUse<T>(T arg) { }
+}

src/tests/JIT/Regression/JitBlue/Runtime_70466/Runtime_70466.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>