I16x8 GtS/GeS/GtU/GeU
I16x8Neg, I8x16Neg
Add Psignb, Psignw, Psubb, Psubw macros

Bug: 
Change-Id: Id38f15140727051e185054295722282b77ea889c
Reviewed-on: https://chromium-review.googlesource.com/773831
Commit-Queue: Jing Bao <jing.bao@intel.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49610}