[clang] [Headers][X86] Add constexpr support for some AVX[512] intrinsics. (PR #158663)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

At the moment we need to keep _mm_mask_cvtepi16_epi8 etc as the selection mask only acts on part of the vector (we're truncating v8i16 to v8i8, using the mask8 on that and passing through all the upper half of the passthrough to create a v16i8) - the best we might be able to do is a custom __builtin_ia32_selectb_64 but it'll be messy - we'll have to come back to it later, maybe when the _mm512_kunpack intrinsics are constexpr?

https://github.com/llvm/llvm-project/pull/158663