This reduction relies on a known object layout of the regexp instance
in order to access the lastIndex field through a statically-determined
offset. Prior to this CL, we checked only for instance types, not for
the map, and thus it was possible to read garbage from either inside
or outside the current object.

Bug: chromium:1024758,v8:7779
Change-Id: I1eec8220797f443bdf3d05804e54f33b21fa2f00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924353Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65039}