[X86][SSE] Add SimplifyDemandedVectorEltsForTargetShuffle to handle target shuffle variable masks

Pulled out from the ongoing work on D66004, currently we don't do a good job of simplifying variable shuffle masks that have already lowered to constant pool entries.

This patch adds SimplifyDemandedVectorEltsForTargetShuffle (a custom x86 helper) to first try SimplifyDemandedVectorElts (which we already do) and then constant pool simplification to help mark undefined elements.

To prevent lowering/combines infinite loops, we only handle basic constant pool loads instead of creating new BUILD_VECTOR nodes for lowering - e.g. we don't try to convert them to broadcast/vzext_load - there might be some benefit to this but if so I'd rather we come up with some way to reuse existing code than reimplement a lot of BUILD_VECTOR code.

Differential Revision: https://reviews.llvm.org/D81791

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