memchr is used in the default case but for two-byte strings that contain
mostly ascii sequences every other byte is 0.

This CL special-cases the 0 byte search in a two-byte string to simply
peform a simple for loop which is rouhly 10x faster than the repeated
memchr approach.

Bug: v8:10737
Change-Id: Icde6b858b6679da69448295616734c46aa95e439
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2364507Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69721}