[clang] [Headers][X86] Allow AVX512 masked arithmetic ss/sd intrinsics to be used in constexpr (PR #162816)

[Simon Pilgrim via cfe-commits]

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@@ -2419,6 +2419,27 @@ static bool interp__builtin_elementwise_int_unaryop(
   return false;
 }
 
+static bool interp__builtin_select_scalar(InterpState &S,
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RKSimon wrote:

Put this next to interp__builtin_select for clarity?

https://github.com/llvm/llvm-project/pull/162816