[clang] [Clang] Add __builtin_selectvector and use it for AVX512 intrinsics (PR #91306)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

I don't understand why the order of emitted instructions changes based on how exactly Clang is compiled, but other than that this should be ready. Hopefully someone spots what the problem could be.

https://github.com/llvm/llvm-project/pull/91306