[X86][SSE] SimplifyDemandedVectorEltsForTargetNode - add general shuffle combining support

This patch uses partial DemandedElts masks to further simplify target shuffle chains and finally starts making target shuffle combining part of SimplifyDemandedBits/SimplifyDemandedVectorElts.

We already manage this for Depth == 0 cases, where combineX86ShuffleChain would early-out if the shuffle combined to the same op, but the patch generalizes this by manipulating the depth handling of combineX86ShufflesRecursively - calling with a new Depth = 0 and reducing the maximum shuffle combine depth accordingly.

Differential Revision: https://reviews.llvm.org/D66004