[clang] [Clang] Add __builtin_selectvector and use it for AVX512 intrinsics (PR #91306)

[Phoebe Wang via cfe-commits]

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@@ -41,6 +41,22 @@ typedef long long __m512i_u __attribute__((__vector_size__(64), __aligned__(1)))
 typedef unsigned char __mmask8;
 typedef unsigned short __mmask16;
 
+#ifdef __cplusplus
+typedef bool __vecmask2 __attribute__((__ext_vector_type__(2)));
+typedef bool __vecmask4 __attribute__((__ext_vector_type__(4)));
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phoebewang wrote:

We used `__mmask8` for `v2i1` and `v4i1` too. I don't we can bitcast them directly.

https://github.com/llvm/llvm-project/pull/91306