Until now, ATOM regexps (i.e. simple patterns don't require regexp matching
logic but can use generic string matching algorithms instead) have always gone
through the slow runtime.

This CL implements a fast path in CSA which simply calls StringIndexOf
internally and then sets up the last-match-info as required.

Local microbenchmarks show a 30% improvement for RE.p.exec on ATOM regexps,
and a 5% improvement on Octane/RegExp.

Bug: v8:6462
Change-Id: I35b4c5caf416fa35fe388dd58e34dea55b098d09
Reviewed-on: https://chromium-review.googlesource.com/535455
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45947}