Previously, we didn't have access checks for the megamorphic case cause
we'd never get to this IC state for a receiver that doesn't hold the
right private field. But now with lazy feedback allocation we share
the megamorphic case code paths for the uninitialized loads as well,
which exposes our bug.

Bug: chromium:982702
Change-Id: I419406bcfc52575260a85d05520c1662735e15f8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1697256Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62668}