[Headers][X86] Allow AVX512fp16 initialization intrinsics to be used in constexpr (#159929)
Fixes https://github.com/llvm/llvm-project/issues/156866
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Signed-off-by: Shreeyash Pandey <shreeyash335@gmail.com>
Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>