[X86] Add support for v16i32 UMUL_LOHI/SMUL_LOHI

Summary: This patch extends the v8i32/v4i32 custom lowering to support v16i32

Reviewers: zvi, RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38274

llvm-svn: 314221