While we only need to check stability of the receiver map if its
inference was "unreliable", we must check stability of each prototype's
map unconditionally.

Bug: chromium:997100
Change-Id: I20071ac9eb74c810ad2ab1d78abfb54a1a006c29
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1768576
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63364}