[llvm] [InstCombine] [X86] pblendvb intrinsics must be replaced by select when possible (PR #137322)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I'm not convinced this all is necessary - a small VectorCombine pass for bitops that matches:
```
  bitop(bitcast(x),bitcast(y)) -> bitcast(bitop(x,y))
```
along with cost checks should be enough.

https://github.com/llvm/llvm-project/pull/137322