Refactor lazy_scan_prune() VM clear logic into helper

Refactor lazy_scan_prune() VM clear logic into helper

Encapsulating the cases that clear the visibility map after vacuum phase
I, when corruption is detected, into in a helper makes the code cleaner
and enables further refactoring in future commits.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/7ib3sa55sapwjlaz4sijbiq7iezna27kjvvvar4dpgkmadml6t%40gfpkkwmdnepx