[X86] isTargetShuffleEquivalent - add BUILD_VECTOR matching

Add similar functionality to isShuffleEquivalent - if the mask elements don't match, try matching the BUILD_VECTOR scalars instead.

As target shuffles need to handle SM_Sentinel values, this can get a bit tricky, so commit just adds actual mask element index handling - full SM_SentinelZero support will be added when the need arises.

Also, enables support in matchVectorShuffleWithPACK

llvm-svn: 369212