This reverts commit 062bb7d4.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [regexp] Limit ATOM regexps to single-character patterns
> 
> 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/589435
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47081}

TBR=yangguo@chromium.org,jgruber@chromium.org

Change-Id: I8655bc4055af5d593f507e16918b434ff45f5379
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6633
Reviewed-on: https://chromium-review.googlesource.com/599547Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47106}