[X86] Add initial SimplifyDemandedVectorEltsForTargetNode support

This patch adds an initial x86 SimplifyDemandedVectorEltsForTargetNode implementation to handle target shuffles.

Currently the patch only decodes a target shuffle, calls SimplifyDemandedVectorElts on its input operands and removes any shuffle that reduces to undef/zero/identity.

Future work will need to integrate this with combineX86ShufflesRecursively, add support for other x86 ops, etc.

NOTE: There is a minor regression that appears to be affecting further (extractelement?) combines which I haven't been able to solve yet - possibly something to do with how nodes are added to the worklist after simplification.

Differential Revision: https://reviews.llvm.org/D52140

llvm-svn: 342564