[X86] Create paddus/psubus from narrower vectors with i8/i16 element types.

Summary:
This patch allows vectors with a power of 2 number of elements and i8/i16 element type to select paddus/psubus instructions. ReplaceNodeResults has been updated to custom widen these operations up to 128 bits like we already do for PAVG.

Another step towards fixing PR38691

Reviewers: RKSimon, spatel

Reviewed By: RKSimon, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51818

llvm-svn: 341753