Swizzles are shuffles that only use values from 1 operand, e.g.
v8x16.shuffle 0 1 2 3 0 0 0 0 4 5 6 7 0 0 0 0 (all the values are < 16).

Match such patterns and emit an optimized codegen that uses less
registers and instructions. Only implemented for x64 for now, the other
backends will come in follow-up patches.

Bug: v8:10696
Change-Id: Iffa694b04c97313eab7d138e4bdad7c0c85cda89
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2335419Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69231}