Use JSObject::FastPropertyAt instead. Also, to avoid adding an
immutable-flag to PropertyAccessInfo, use DataConstant (instead of
DataField) for properties that are immutable according to their
attributes.

This is in preparation for serializing the property value for
concurrent inlining.

Bug: v8:7790
Change-Id: Ib40059bde2e5eb14b26400bcab72d6ea6bb57666
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624790Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61747}