Keep all_frozen updated in heap_page_prune_and_freeze

Keep all_frozen updated in heap_page_prune_and_freeze

Previously, we relied on all_visible and all_frozen being used together
to ensure that all_frozen was correct, but it is better to keep both
fields updated.

Future changes will separate their usage, so we should not depend on
all_visible for the validity of all_frozen.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com