The logic is different for shared and non-shared memory, so it's
cleaner to have different functions for them.

Bug: v8:11111
Change-Id: I95d43b54c207b8059ea59d6d0f873623de946be3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2903152
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74656}