Fix #3821: keep ref-struct conditional as if/return, not ?. / ?? (CS8978)

[sailro]

Fixes #3821.

NullPropagationTransform rewrote testedVar != null ? testedVar.AccessChain : default
to testedVar?.AccessChain ?? default whenever the access-chain result was a
non-nullable value type. For a by-ref-like type (a ref struct such as Span<T>)
this does not compile: a ref struct cannot be wrapped in Nullable<T> (CS8978).

This excludes by-ref-like return types from the null-coalescing rewrite, leaving
the conditional as an if/return, which compiles.

Test

A Correctness/NullPropagation case returns a ref struct from a
holder != null ? holder.Get() : default conditional (guarded #if CS72).
Without the fix the decompiled holder?.Get() ?? default fails to recompile
(CS8978); with it the decompiled output recompiles and behaves identically.

Verified against a build with this change by decompiling a minimal assembly:
h != null ? h.GetSpan() : default now decompiles to an if/return instead of
h?.GetSpan() ?? default(Span<byte>).

[Copilot]

Pull request overview

This PR fixes an invalid null-propagation rewrite in NullPropagationTransform that produced uncompilable C# when the access-chain returns a by-ref-like value type (e.g., a ref struct such as Span<T>), addressing #3821.

Changes:

• Exclude by-ref-like return types from the ?. / ?? null-coalescing rewrite that relies on nullable-lifting.
• Add a correctness test case (guarded by #if CS72) that returns a ref struct from holder != null ? holder.Get() : default to ensure the decompiled output remains recompilable.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ICSharpCode.Decompiler/IL/Transforms/NullPropagationTransform.cs	Adds a guard (!returnType.IsByRefLike) to prevent emitting ?./?? for by-ref-like return types that cannot be made nullable.
ICSharpCode.Decompiler.Tests/TestCases/Correctness/NullPropagation.cs	Adds a C# 7.2+ test case covering ref struct return values to prevent regressions.

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[siegfriedpammer]

Thank you for your contribution!