BigInts are immutable after initialization, thus safe to access
concurrently if the Heap::IsPendingAllocation predicate returns false.

Bug: v8:7790
Change-Id: Icf3954b1e50af2132846eca5304c7947afb8f298
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2856535
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74322}