Allow the disasm output on xarch to do less work for the common path

[tannergooding]

Most hardware we run on is currently VEX capable and we can spend a decent amount of time simply copying the instruction names into a temporary buffer just to add the v prefix.

This cleans that up by defaulting the instruction tables to have the v prefixed name already and to instead simply return that + 1 in the case we're in a legacy hardware scenario.

It also makes it easier to add instructions with pseudo names in the future.

[dotnet-policy-service]

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[Copilot]

Pull Request Overview

This PR refactors the disassembly output path for x86 instructions by precomputing pseudo-names for instructions, reducing unnecessary string copying in the common VEX path and simplifying the legacy hardware fallback.

• Introduces a new flag (INS_FLAGS_HasPseudoName) in the instruction flags.
• Updates display name generation in instr.cpp to remove redundant prefix operations.
• Refactors checks for pseudo-names in emitxarch.cpp and exposes the helper via codegeninterface.h.

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/instr.h	Added a new flag to mark instructions with a pseudo display name.
src/coreclr/jit/instr.cpp	Refactored display name functions with new lambdas and logic.
src/coreclr/jit/emitxarch.cpp	Simplified dispatch for instructions with pseudo names.
src/coreclr/jit/codegeninterface.h	Declared the new instHasPseudoName helper.

[tannergooding]

CC. @dotnet/jit-contrib, small refactoring to make the instruction name fixups easier to understand and maintain. Reduces the amount of string copying for a typical jit disasm output by a decent amount as well.