Since we have an uninitialized TransitionArray that we want to insert
an element (map1), we can't guarantee that said element would exist at
the point of the search. Then, we search for an element guaranteed not
to be (map2) and we check that we did not find it.

If we have a data race, this would also trigger it.

Bug: v8:7790
Change-Id: Ib90044d7c0901d599aed041f608f2c0bce506d67
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2319995
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69099}