pgsql: 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(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/7ib3sa55sapwjlaz4sijbiq7iezna27kjvvvar4dpgkmadml6t%40gfpkkwmdnepx

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/19af794b660e3711d8b698aeedfc33e13dc235d8

Modified Files
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src/backend/access/heap/vacuumlazy.c | 131 ++++++++++++++++++++++-------------
1 file changed, 83 insertions(+), 48 deletions(-)