Currently the FeedbackVector uses arrays in slots that are polymorphic,
usually in a <map, handler> tuple pattern. Helper functions try to
re-use an existing array if it's already in place.

For Concurrent TurboFan, it would be far better if these FixedArrays
were immutable. We could then count on semantic correctness when
harvesting their information from a background thread without locking.

Additionally, the arrays should always be initialized fully before
being set in place.

Bug: v8:7790
Change-Id: I81eae3bda48c2d0d8eea41d1bc9c62afb7e619d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264364Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68532}