In fast mode, this CL try to avoid calling substring in @@match.
For an ATOM type regexp, hold the literal string to search for before the loop
and reuse the string instead of calling substring in the loop.

Change-Id: Ice314ebf146261cf206c21cb1530a2a44d3c42ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1618435Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61763}