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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The relevant bit of code is:

```
  // The OpenCL operator with a vector condition is sufficiently
  // different to merit its own checker.
  if ((getLangOpts().OpenCL && Cond.get()->getType()->isVectorType()) ||
      Cond.get()->getType()->isExtVectorType())
    return OpenCLCheckVectorConditional(*this, Cond, LHS, RHS, QuestionLoc);
```

Maybe makes sense to relax it.

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