This reverts commit 10817205.

While increasing the number of IRREGEXP regexp instances (vs. ATOM)
gives us a 3% perf improvement, it also results in higher memory
overhead. This CL is the suspected culprit for the recent 5x increase
in OOM crashes from within regexp codegen.

Bug: v8:6633, chromium:790833
Change-Id: Icca70b31fbda8cfb7a63dc895f6665dfe534359d
Reviewed-on: https://chromium-review.googlesource.com/817294Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49963}