[X86] SimplifyDemandedVectorEltsForTargetNode - remove identity target...

[X86] SimplifyDemandedVectorEltsForTargetNode - remove identity target shuffles before simplifying inputs

By removing demanded target shuffles that simplify to zero/undef/identity before simplifying its inputs we improve chances of further simplification, as only the immediate parent user of the combined is added back to the work list - this still doesn't help us if its passed through other ops though (bitcasts....).

llvm-svn: 343390