By marking maps detached from the transition tree as prototypes, we'll
automatically stop tracking transitions from those detached fast maps. That
allows us to quickly check whether a map is detached (or the initial map
anyway); and saves memory. We can use this information to ignore sibling type
feedback when parsing a JSON array with many distinctly shaped json objects.

Bug: chromium:993980
Change-Id: I86d493ac2cabec2c31c6e322ad5c5a7ace059dfc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1771778Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63403}