JIT: Strip GC/reloc flags from the size in arm64 instGen_Set_Reg_To_Imm

[jakobbotsch]

The immediate encoding helpers (emitIns_valid_imm_for_mov, canEncodeHalfwordImm, canEncodeBitMaskImm) take a plain 4/8 byte data size and assert on anything else. Every other caller strips the flags first, but instGen_Set_Reg_To_Imm only did so when relocations were disabled, so with relocations enabled a non-null byref constant reached them as EA_BYREF and tripped

Assertion failed 'isValidGeneralDatasize(size)' in emitarm64.cpp:3025

The movz/movk fallback had the same problem: its 'size == EA_8BYTE' tests would compute a 32 bit width for EA_BYREF and drop the upper halfwords.

Only reproduces when compiling ahead of time (crossgen2/NativeAOT), where compReloc is set.

Fixes #132268

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[Copilot]

Pull request overview

Fixes an arm64 JIT correctness bug where instGen_Set_Reg_To_Imm could pass an emitAttr with GC/reloc flags (e.g., EA_BYREF) into immediate-encoding helpers that require a plain 4/8-byte size, causing assertions (and incorrect movz/movk width decisions) when relocations are enabled (AOT scenarios).

Changes:

• In CodeGen::instGen_Set_Reg_To_Imm, compute immSize = EA_SIZE(size) and use it for emitIns_valid_imm_for_mov and the movz/movn/movk fallback width decisions.
• Add a new JIT regression test for issue #132268.
• Register the new test in Regression_ro_2.csproj.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/codegenarm64.cpp	Strips flags from the size used by immediate-encoding decisions while preserving full emitAttr for emission/reloc handling.
src/tests/JIT/Regression/Regression_ro_2.csproj	Includes the new JitBlue regression test source file in the merged regression project.
src/tests/JIT/Regression/JitBlue/Runtime_132268/Runtime_132268.cs	Adds a repro-based xUnit regression test targeting the byref-constant path that triggered the arm64 AOT assertion.