[X86] Lower v16i16->v8i16 truncate using an 'and' with 255, an...

[X86] Lower v16i16->v8i16 truncate using an 'and' with 255, an extract_subvector, and a packuswb instruction.

Summary: This is an improvement over the two pshufbs and punpcklqdq we'd get otherwise.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54671

llvm-svn: 347171