In the process, cleanup the StoreFixedArray* operators
and change most FixedArray element accesses so that
they explicitly use the '.objects' and '.floats'
fields.

Bug: v8:7793
Change-Id: I3e45a9b7536ec76e1413b7e508d79a56b37604ff
Reviewed-on: https://chromium-review.googlesource.com/c/1460948
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59649}