Combine FSM updates for prune and no-prune cases.
lazy_scan_prune() and lazy_scan_noprune() update the freespace map
with identical conditions; combine them. This consolidation is easier
now that
cb970240f13df2b63f0410f81f452179a2b78d6f moved visibility map
updates into lazy_scan_prune().
While combining the FSM updates, simplify the logic for calling
lazy_scan_new_or_empty() and lazy_scan_noprune().
Also update a few comemnts in this part of the code to make them,
hopefully, clearer.
Melanie Plageman and Robert Haas
Discussion: https://postgr.es/m/CA%2BTgmoaLTvipm%3Dxx4rJLr07m908PCu%3DQH3uCjD1UOn8YaEuO2g%40mail.gmail.com