Since invalidation is disabled at the moment, slots still need to be
cleared explicitly. This was not the case in MigrateFastToFast in
case a slot transitioned from tagged to untagged.

Bug: chromium:1006630
Change-Id: I8d32ab9196603dabad9a382ec49b39144624c30d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815248Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63929}