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

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

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

No, I mean I have a vector of bools and want to select a value based on that. e.g. `declval<simd_vector<bool, 16>>() ? declval<simd_vector<int, 16>>() : declval<simd_vector<int, 16>>()`.

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