There's an inherent trade-off when deciding between ATOM and IRREGEXP
regexps: IRREGEXP is faster at runtime for all but trivial single-character
patterns, while ATOM regexps have a lower memory overhead.

This CL is intended to help investigate impact on benchmarks and real-world
code - if something tanks, it's easy to revert, otherwise it can be a first
step towards a possible removal of ATOM regexps.

Bug: v8:6633
Change-Id: Ia41d8eb28d33952735562d3d4127202746a6ac4e
Reviewed-on: https://chromium-review.googlesource.com/589435Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47081}