Add helper for freeze determination to heap_page_prune_and_freeze

Add helper for freeze determination to heap_page_prune_and_freeze

After scanning the line pointers on a heap page during the first phase
of vacuum, we use the information collected to decide whether to use
the assembled freeze plans.

Move this decision logic into a helper function to improve readability.

While here, rename a PruneState member and disambiguate some local
variables in heap_page_prune_and_freeze().

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/2wk7jo4m4qwh5sn33pfgerdjfujebbccsmmlownybddbh6nawl%40mdyyqpqzxjek