Now that fields are always in the same order as descriptors, we can
change the property scan during NextFreePropertyIndex into a faster
reverse search that simply tries to find the last field.

Change-Id: I24d0781cd7d7c5b15998c35f281be69cd492d5ee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667402Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62286}