pgsql: Add helper for freeze determination to heap_page_prune_and_freez

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(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/2wk7jo4m4qwh5sn33pfgerdjfujebbccsmmlownybddbh6nawl%40mdyyqpqzxjek

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c8dd6542bae42744c676234dca45972e2cc3dfb2

Modified Files
--------------
src/backend/access/heap/pruneheap.c | 221 +++++++++++++++++++++---------------
1 file changed, 131 insertions(+), 90 deletions(-)