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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

clang already supports `?:` with a vector condition; does this add anything new on top of that?

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