As per (https://github.com/tc39/proposal-string-matchall/pull/35), the
call to IsRegExp after CreateRegExp was removed and additional
checking was replaced by an Assert.

Updates to Test262 has been submitted:
https://github.com/tc39/test262/pull/1587

Bug: v8:6890
Change-Id: I942b6846bb46cf85b1ea5566f9c19de7d2dbf03e
Reviewed-on: https://chromium-review.googlesource.com/1086419Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Cr-Commit-Position: refs/heads/master@{#53539}