[Headers][X86] Allow AVX512F mullox intrinsics to be used in constexpr (#156722)

This PR adds constexpr support for the following AVX512F mullox
intrinsics:

  - _mm512_mullox_epi64
  - _mm512_mask_mullox_epi64

Closes #156632
Fixes #155411

Author: markbhasawut

clang/lib/Headers/avx512fintrin.h

@@ -1463,12 +1463,12 @@ _mm512_mask_mullo_epi32(__m512i __W, __mmask16 __M, __m512i __A, __m512i __B) {
                                              (__v16si)__W);
 }
 
-static __inline__ __m512i __DEFAULT_FN_ATTRS512
-_mm512_mullox_epi64 (__m512i __A, __m512i __B) {
+static __inline__ __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
+_mm512_mullox_epi64(__m512i __A, __m512i __B) {
   return (__m512i) ((__v8du) __A * (__v8du) __B);
 }
 
-static __inline__ __m512i __DEFAULT_FN_ATTRS512
+static __inline__ __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_mask_mullox_epi64(__m512i __W, __mmask8 __U, __m512i __A, __m512i __B) {
   return (__m512i)__builtin_ia32_selectq_512((__mmask8)__U,
                                              (__v8di)_mm512_mullox_epi64(__A, __B),

clang/test/CodeGen/X86/avx512f-builtins.c

@@ -3171,13 +3171,15 @@ __m512i test_mm512_mullox_epi64 (__m512i __A, __m512i __B) {
   // CHECK: mul <8 x i64>
   return (__m512i) _mm512_mullox_epi64(__A, __B);
 }
+TEST_CONSTEXPR(match_v8di(_mm512_mullox_epi64((__m512i)(__v8di){+1, -2, +3, -4, +5, -6, +7, -8}, (__m512i)(__v8di){-2, +3, +4, +5, -6, +7, +8, +9}), -2, -6, +12, -20, -30, -42, +56, -72));
 
 __m512i test_mm512_mask_mullox_epi64 (__m512i __W, __mmask8 __U, __m512i __A, __m512i __B) {
   // CHECK-LABEL: test_mm512_mask_mullox_epi64
   // CHECK: mul <8 x i64> %{{.*}}, %{{.*}}
   // CHECK: select <8 x i1> %{{.*}}, <8 x i64> %{{.*}}, <8 x i64> %{{.*}}
   return (__m512i) _mm512_mask_mullox_epi64(__W, __U, __A, __B);
 }
+TEST_CONSTEXPR(match_v8di(_mm512_mask_mullox_epi64((__m512i)(__v8di){-100, +200, -300, +400, -500, +600, -700, +800}, 0x00FF, (__m512i)(__v8di){+1, -2, +3, -4, +5, -6, +7, -8}, (__m512i)(__v8di){-2, +3, +4, +5, -6, +7, +8, +9}), -2, -6, +12, -20, -30, -42, +56, -72));
 
 __m512d test_mm512_add_round_pd(__m512d __A, __m512d __B) {
   // CHECK-LABEL: test_mm512_add_round_pd