[X86][SSE] matchVectorShuffleAsBlend - use Zeroable element mask directly.

We can make use of the Zeroable mask to indicate which elements we can safely set to zero instead of creating a target shuffle mask on the fly.

This allows us to remove createTargetShuffleMask.

This is part of the work to fix PR43024 and allow us to use SimplifyDemandedElts to simplify shuffle chains - we need to get to a point where the target shuffle masks isn't adjusted by its source inputs in setTargetShuffleZeroElements but instead we cache them in a parallel Zeroable mask.

llvm-svn: 373846