[X86] matchShuffleWithSHUFPD - use Zeroable element mask directly. NFCI.

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 only leaves one user of createTargetShuffleMask which we can hopefully get rid of in a similar manner.

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: 373641

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