Re: [PATCH] Fix references in comments, and sync up heap_page_is_all_visible() with heap_page_prune_and_freeze()

While working on [1] I found an outdated comment in heap_page_is_all_visible() and two other small fixes.

0001: Updates that comment so future authors know that this "stripped down function" should retain the logic in heap_page_prune_and_freeze(), not lazy_scan_prune() as was the case before 6dbb490.
0002: Mimics the same loop logic as in heap_page_is_all_visible() so as to a) stay in sync and b) benefit from the mentioned CPU prefetching optimization.
0003: Moves the ItemSetPointer() just a bit further down in the function again to a) stay in sync and b) to sometimes avoid that tiny overhead.

best,

-greg

PS: per-community standards I've switched to my personal email address rather than gregburd@amazon.com

[1] https://www.postgresql.org/message-id/flat/78574B24-BE0A-42C5-8075-3FA9FA63B8FC@amazon.com