[X86][SSE] Utilize ZeroableElements for canWidenShuffleElements

canWidenShuffleElements can do a better job if given a mask with ZeroableElements info. Apparently, ZeroableElements was being only used to identify AllZero candidates, but possibly we could plug it into more shuffle matchers.

Original Patch by Zvi Rackover @zvi

Differential Revision: https://reviews.llvm.org/D42044

llvm-svn: 336903