[llvm] Add option to turn off optimization for X86 assembler (PR #75895)

[Yi Kong via llvm-commits]

kongy wrote:

> The intention is not quite clear to me. For example, if you just want to always get VEX3 encoding, you can use `{vex3}` before the instruction. It is officially supported by both llvm-mc and gas. It looks less optimization than assembler preference to me.

We have tests that use clang assembler as a verification tool, to verify that our assembler generates the correct (i.e. the same) output. Therefore, clang assembler performing optimisations is undesired.

https://github.com/llvm/llvm-project/pull/75895