[clang] [Headers][X86] Enable constexpr handling for MMX/SSE/AVX/AVX512 avg intrinsics (PR #157464)

[Simon Pilgrim via cfe-commits]

================
@@ -690,50 +690,40 @@ _mm512_maskz_adds_epu16 (__mmask32 __U, __m512i __A, __m512i __B)
                                         (__v32hi)_mm512_setzero_si512());
 }
 
-static __inline__ __m512i __DEFAULT_FN_ATTRS512
-_mm512_avg_epu8 (__m512i __A, __m512i __B)
-{
-  return (__m512i)__builtin_ia32_pavgb512((__v64qi)__A, (__v64qi)__B);
+static __inline__ __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
+_mm512_avg_epu8(__m512i __A, __m512i __B) {
+  return (__m512i)__builtin_ia32_pavgb512((__v64qu)__A, (__v64qu)__B);
 }
 
-static __inline__ __m512i __DEFAULT_FN_ATTRS512
-_mm512_mask_avg_epu8 (__m512i __W, __mmask64 __U, __m512i __A,
-          __m512i __B)
-{
-  return (__m512i)__builtin_ia32_selectb_512((__mmask64)__U,
-              (__v64qi)_mm512_avg_epu8(__A, __B),
-              (__v64qi)__W);
+static __inline__ __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
+_mm512_mask_avg_epu8(__m512i __W, __mmask64 __U, __m512i __A, __m512i __B) {
+  return (__m512i)__builtin_ia32_selectb_512(
+      (__mmask64)__U, (__v64qu)_mm512_avg_epu8(__A, __B), (__v64qu)__W);
----------------
RKSimon wrote:

__builtin_ia32_selectb_512 still takes __v64qi types - so you need to keep the original types:
```
  return (__m512i)__builtin_ia32_selectb_512(
      (__mmask64)__U, (__v64qi)_mm512_avg_epu8(__A, __B), (__v64qi)__W);
```
same for the other mask/maskz intrinsics

https://github.com/llvm/llvm-project/pull/157464