Previously the fast path only asserted the correct instance types; but
when reading lastIndex we additionally rely on a specific object
shape.  This is checked by HasInitialRegExpMap().

Bug: chromium:1024758
Change-Id: I0b401ffb246dd47153caf798446d8d41bc84bc8e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924354
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65071}