[X86] Add support for turning vXi1 shuffles into KSHIFTL/KSHIFTR.

This patch recognizes shuffles that shift elements and fill with zeros. I've copied and modified the shift matching code we use for normal vector registers to do this. I'm not sure if there's a good way to share more of this code without making the existing function more complex than it already is.

This will be used to enable kshift intrinsics in clang.

Differential Revision: https://reviews.llvm.org/D51401

llvm-svn: 341227