[clang] [X86] Allow PSHUFD/PSHUFLW/PSHUFW intrinsics in constexpr. (PR #161210)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon requested changes to this pull request.

Thanks for this - the shuffle decode looks like it can be simplified quite a bit

https://github.com/llvm/llvm-project/pull/161210