[clang] [Headers][X86] Allow SSE2/AVX2/AVX512F/AVX512BW/AVX512DQ integer arithmetic intrinsics to be used in constexpr (PR #157582)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

> @RKSimon from what I can see in clang/lib/Headers/mmintrin.h, it looks like all the intrinsics you mentioned are already being used in CONSTEXPR, shall I make test cases for it?

Ah! I forgot I'd done these ages ago - they should already have test coverage if I did it properly. Thanks for checking.

https://github.com/llvm/llvm-project/pull/157582