If b is S128Zero, Shuffle(a,b,s) can be optimized to
Swizzle(a,s). By setting s[i] to 0x80, we can avoid access b.
If a is S128Zero, we can swap a and b first.

If one input of I8x16Shuffle is S128Zero, this patch can save
~60% instructions(7 of 12), and more than 30% improvement is
observed in local microbenchmarks.

Change-Id: I5953fa9064e01203cd4cf423c55dd5ed33cad57e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3544992Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Jie Pan <jie.pan@intel.com>
Cr-Commit-Position: refs/heads/main@{#80623}