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