[clang] [X86][Clang] Support constexpr evaluation of cvtpd2ps intrinsics (PR #169980)

[Simon Pilgrim via cfe-commits]

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@@ -207,9 +207,8 @@ _mm512_undefined(void)
   return (__m512)__builtin_ia32_undef512();
 }
 
-static __inline__ __m512 __DEFAULT_FN_ATTRS512
-_mm512_undefined_ps(void)
-{
+static __inline__ __m512 __DEFAULT_FN_ATTRS512_CONSTEXPR
+_mm512_undefined_ps(void) {
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RKSimon wrote:

All the avx512 pd2ps intrinsics look like they're going to be a headache due to reuse of more general builtins - I think for this first patch you'd be better off dropping them and just deal with the the SSE2/AVX builtins constexpr handling.

https://github.com/llvm/llvm-project/pull/169980