Rename and relocate freeze plan dedup routines.

Rename and relocate freeze plan dedup routines.

Rename the heapam.c freeze plan deduplication routines added by commit
9e540599 to names that follow conventions for functions in heapam.c.
Also relocate the functions so that they're next to their caller, which
runs during original execution, when FREEZE_PAGE WAL records are built.

The routines were initially placed next to (and followed the naming
conventions of) conceptually related REDO routine code, but that scheme
turned out to be kind of jarring when considered in a wider context.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20230109214308.icz26oqvt3k2274c@awork3.anarazel.de