Flag getters (e.g. RegExp.p.get global) are defined on the prototype and
thus we need to use the more general BranchIfFastRegExp here instead of
IsFastRegExpNoPrototype.

Bug: chromium:800538
Change-Id: Ib6bc8a4fd3bf2f7dd31538c8dbb61814106c184b
Reviewed-on: https://chromium-review.googlesource.com/859767Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50538}