Combine FSM updates for prune and no-prune cases.

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