Don't deopt when a PropertyCell changes from readonly to writable.
Turbofan doesn't depend on readonly-ness unless the property is
also non-configurable, in which case such a change can't happen.

Change-Id: I3d1078a8adf1ec1b16d973dd71c4295d71003a8d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531791
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71184}