[X86] Use BWI instructions to improve lowering of v32i8 MULHU/S

Summary: If we have BWI instructions we can widen to v32i16 to do the multiply instead of splitting.

Reviewers: RKSimon, spatel, zvi

Reviewed By: zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38305

llvm-svn: 314432