[compiler] AssumeMemoryFence for a few refs

.. namely:

 AllocationSite::nested_site
 CodeHandlerInfo::data
 ScopeInfo::OuterScopeInfo

These are all immutable after initialization.

Bug: v8:7790,chromium:1237387
Change-Id: I73f1c366d9f4fa9ad721051dea668227ba987e63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3080559


Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76152}