I8x16Shl/I8x16ShrS/I8x16ShrU,I8x16Mul

R=bbudge@chromium.org, bmeurer@chromium.org

Bug: 
Change-Id: I97d7f077c26fe6f8be6464582f20d4e3c8fd4667
Reviewed-on: https://chromium-review.googlesource.com/853772
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50940}