[clang] [llvm] [X86][MC,Driver] Support -msse2avx to encode SSE instruction with VEX prefix (PR #96860)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

This driver option needs a `TargetSpecific` flag, otherwise this LGTM.

https://github.com/llvm/llvm-project/pull/96860