This fixes a spec bug in which the order of calls to 1) the flag getter
and 2) ToUint32(limit) was incorrect if ToUint32 pushes the regexp
instance onto the slow path. We are now more restrictive and completely
avoid ToUint32 on the fast path.

Bug: chromium:801171
Change-Id: I21d15fe566754d2bc05853f895636bb882fbf599
Reviewed-on: https://chromium-review.googlesource.com/863644Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50533}