Detect AVX2 support and use vpbroadcastb or vpbroadcastw.

No new assembler helpers required because we are only emitting the
VEX-128 versions of these instructions.

Bug: v8:11258
Change-Id: Ic50178daa6fc8fe767dfc788e61e67538066bdea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2596582
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71866}