Add a DCHECK during deserializer allocation that the previous allocation
is sufficiently initialized to be iterable. This is an step towards
allowing GC during deserializer execution.

Bug: v8:10815
Change-Id: I29da21b93e6b826bdb7b5f9f5a9723da1698a225
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2396079
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69802}