This optimizes i8x16 shifts when shift value is constant. It reduces
instruction counts from 10 to 6 (unsigned), and 9 to 5 (signed).

We can use a word (16-bit) shift, then mask away the high (shru) or low
(shl) bits to achieve a byte shift. Most of the instructions are
dedicated to building the mask.

Bug: v8:10115
Change-Id: Ie602c2b0a7227181502fadb14e100bb0b92f322f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2103445
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66755}