Specialize __GetFieldHelper per generic instantiation

[MichalStrehovsky]

We have an optimization that tries to avoid bringing in constructed MethodTable for struct fields that are reference types. However, if the type of the field is T on a generic type, we had to be conservative because we didn't know if it was a valuetype or not. Use SpecializableILStubMethod instead of ILStubMethod so we know.

Cc @dotnet/ilc-contrib

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[Copilot]

Pull Request Overview

This PR refactors the value-type field helper stub to support per-instantiation specialization and avoids unnecessary MethodTable creation for generic parameters. Key changes include:

• Switched the ValueTypeMethodHashtable from DefType to MetadataType for broader type keying.
• Updated ValueTypeGetFieldHelperMethodOverride to inherit from SpecializableILStubMethod and split IL emission into open and specialized paths.
• Introduced common EmitILCommon to share most of the emitter logic.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/CompilerTypeSystemContext.ValueTypeMethods.cs	Changed hashtable key and method signatures from DefType to MetadataType.
src/coreclr/tools/Common/TypeSystem/IL/Stubs/ValueTypeGetFieldHelperMethodOverride.cs	Converted stub to SpecializableILStubMethod, added overload for specialized IL emission, and centralized IL generation in EmitILCommon.

Comments suppressed due to low confidence (1)

src/coreclr/tools/Common/TypeSystem/IL/Stubs/ValueTypeGetFieldHelperMethodOverride.cs:65

• Add unit tests for ValueTypeGetFieldHelperMethodOverride covering both the open and specialized IL emission paths, including scenarios with signature-variable fields and inline arrays to ensure correctness after these refactorings.

        public override MethodIL EmitIL(MethodDesc specializedMethod)

[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

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[jkotas]

Thanks