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

[Phoebe Wang 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.
+
+**Signature**:
+
+.. code-block:: c++
+
+  template <class T, size_t N>
+  simd_vec<T, N> __builtin_selectvector(simd_vec<T, N> lhs, simd_vec<T, N> rhs,
+                                        simd_vec<bool, N> cond)
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phoebewang wrote:

Maybe put `cond` the first operand to match with `select` and the old X86 builtins?

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