This is a reland of 76a2ab06

Changes since the original CL:
- Handle unserialized elements (optional result in getter).
- Merge should_access_heap and --turbo-direct-heap-access paths.
- Slightly update the serialized path in GetOwnCowElement.
- Fix the cctest, add a regression test.

Atomic JSObject::elements/JSArray::length setters are addressed
in this CL: crrev.com/c/2704076.

Original change's description:
> [compiler] Direct heap reads for JSArrayRef
>
> There are two aspects to the non-JSObject parts of JSArrayRef:
>
> - JSArrayRef::length. Relevant only in two spots, 1. when reading
> (immutable) array boilerplates and 2. for GetOwnCowElement.
>
> - JSArrayRef::GetOwnCowElement. May read into a copy-on-write backing
> store. Relies on the invariant that cow backing stores are immutable.
>
> This CL renames the length accessor to length_unsafe to make the
> danger explicit at callsites.
>
> For GetOwnCowElement the refactor is slightly larger, since we now
> need to read into the backing store while keeping full control of
> object reads (e.g. JSArray::length and JSArray::elements_kind). We
> make all reads explicit at the call site by requiring that elements,
> elements kind, and length are passed in as arguments to
> GetOwnCowElement. Inside GetOwnCowElement, consistency between these
> is *not* guaranteed due to concurrency. At runtime, consistency *is*
> guaranteed through the reference-equality check on the elements seen
> during compilation. The actual elements read is implemented in
> ConcurrentLookupIterator::GetOwnCowElement.
>
> Bug: v8:7790
> Change-Id: I9aa169ce4f2b1e2bfe1e9232007669eb7654a995
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2695403
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72834}

Bug: v8:7790
Change-Id: I7577ad554992cafff81099a28c34f27db9bd8042
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2710431
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72904}