Minor differences in how we dispatch on the regexp type
(IRREGEXP,ATOM,NOT_COMPILED) make significant differences in benchmark
performance. A simple switch turns out to be the best alternative.

BUG=chromium:734035, v8:6462

Change-Id: I09c613658e828b9fd1e3082624ef692b8b4a0c5f
Reviewed-on: https://chromium-review.googlesource.com/539295Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45998}