Existing tests already exercise KeyedStoreIC; this CL makes them
take the fast path instead of the slow path.

Bug: v8:11111
Change-Id: I13b84650ae37abe1a8a46d53c9a9b7104bed31de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2910315
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74706}