[APX] refactor APX-EVEX emitter

[Ruihan-Yin]

Contribute to #117326

Overview of this PR:
As tanner suggested, the first goal for this PR is to get rid of TakesApxExtendedEvexPrefix and let TakesEvexPrefix manage all the cases when EVEX is needed, both original SIMD EVEX, and extended EVEX introduced by APX.

The expected outcome is to reach better readability, and better robustness. meanwhile have little to no impact on throughput.

After the PR:
TakesEvexPrefix will gate all the EVEX use cases, and IsEvexEcodableInstruction will confirm the EVEX encodability for both SIMD and APX instruction. This makes fewer places that need to be updated if more APX instructions or feature comes.

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[Copilot]

Pull request overview

This PR refactors xarch emitter/APX encoding classification by introducing an explicit Encoding_APX_EVEX flag and consolidating EVEX gating so APX “extended EVEX” cases flow through the existing TakesEvexPrefix / IsEvexEncodableInstruction logic rather than a separate TakesApxExtendedEvexPrefix path.

Changes:

• Adds Encoding_APX_EVEX (and an INS_FLAGS_APX_EVEX_PROMOTABLE mask) to mark APX EVEX-only / EVEX-promotable instructions in the instruction metadata.
• Removes TakesApxExtendedEvexPrefix and updates prefix selection/sizing logic to rely on TakesEvexPrefix, with SIMD-only queries split into IsSimdVexOrEvexEncodableInstruction and IsSimdEvexEncodableInstruction.
• Updates EVEX encodability and EVEX prefix emission to correctly handle APX-promoted legacy/VEX/APX-only instructions, including MAP4 handling for legacy/APX-only EVEX promotion.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/instrsxarch.h	Marks APX-only/APX EVEX-form instructions with Encoding_APX_EVEX to drive unified EVEX gating.
src/coreclr/jit/instr.h	Introduces Encoding_APX_EVEX and INS_FLAGS_APX_EVEX_PROMOTABLE to classify APX EVEX promotion capability.
src/coreclr/jit/emitxarch.h	Removes TakesApxExtendedEvexPrefix declaration and adds SIMD-scoped EVEX/VEX query helpers.
src/coreclr/jit/emitxarch.cpp	Refactors EVEX encodability/prefix selection and updates sizing/output paths to account for APX-promoted EVEX behavior.

[Ruihan-Yin]

Build analysis is green, and copliot left no comments.

Hi @tannergooding, can you please take a look at this PR? Thanks! :)

[JulieLeeMSFT]

LGTM.
Traced the flag checks, prefix selection, and stress-mode handling against the source — behavior is preserved and the cleanup is solid. Removing TakesApxExtendedEvexPrefix() and routing everything through IsEvexEncodableInstruction()/TakesEvexPrefix() via explicit metadata flags is a nice simplification. KMOV/BMI correctly moved to DoJitStressPromotedEvexEncoding() (MAP4, not standard VEX→EVEX), and the Map-4 prefix logic + comment are correct. No blocking concerns.

🤖 AI-generated code review

[JulieLeeMSFT]

CC @dhartglassMSFT.