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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

You mean, if all three operands are boolean vectors?  I'm surprised that doesn't already work.

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