The actual fix is in LoadIC::ComputeHandler (checking
lookup_start_object == holder instead of receiver == holder) + the
LookupIterator changes for preserving lookup_start_object.

The rest is renaming / refactoring.

Reland: not relying on the prototype validity cell after all

Previous version: https://chromium-review.googlesource.com/c/v8/v8/+/2414039

Bug: v8:9237, chromium:1127653
Change-Id: I1949442f8ddcecb776f0c5d2cf737cb75f80e313
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2428588Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70112}