[X86][SSE] SimplifyDemandedVectorEltsForTargetNode - add faux shuffle support.

This patch decodes target and faux shuffles with getTargetShuffleInputs - a reduced version of resolveTargetShuffleInputs that doesn't resolve SM_SentinelZero cases, so we can correctly remove zero vectors if they aren't demanded.

llvm-svn: 371353