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

For Signed, we 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: I1d5c0e0fb779eeb7e0185d3cb7fd595837fd8daf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2106293Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66779}