To address previously observed regressions, this CL also introduces
unchecked FixedArray accessors and uses them to access collections.

Bug: v8:8029
Change-Id: I6bcd8db2b89b29b7acb3b8431ec5405b737bcef2
Reviewed-on: https://chromium-review.googlesource.com/c/1473033
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59631}