This CL unifies performance-critical slot processing code that was manually
specifaized for ObjectSlot. Now one templated implementation can be used
for processing both ObjectSlot and MaybeObjectSlot.

Bug: v8:8518
Change-Id: Ia4346a817911f8042459ce579741fe2308ef5e4d
Reviewed-on: https://chromium-review.googlesource.com/c/1354459
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57933}