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

[Simon Pilgrim via cfe-commits]

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@@ -3019,6 +3019,26 @@ C-style cast applied to each element of the first argument.
 
 Query for this feature with ``__has_builtin(__builtin_convertvector)``.
 
+``__builtin_selectvector``
+--------------------------
+
+``__builtin_selectvector`` is used to express generic vector element selection.
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RKSimon wrote:

Extend this description to explicitly describe the input/output types and mechanism - don't just rely on the code snippet (although that's a nice accompaniment): The input must all be vectors of the same same number of elements, the 2 first operands must be the same type etc. etc. (basically everything in SemaChecking).

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