pgsql: Split heap_page_prune_and_freeze() into helpers

Split heap_page_prune_and_freeze() into helpers

Refactor the setup and planning phases of pruning and freezing into
helpers. This streamlines heap_page_prune_and_freeze() and makes it more
clear when the examination of tuples ends and page modifications begin.

No code change beyond what was required to extract the code into helper
functions.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/mhf4vkmh3j57zx7vuxp4jagtdzwhu3573pgfpmnjwqa6i6yj5y%40sy4ymcdtdklo

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e135e044572e88291343f45ce65b406ac963b0e7

Modified Files
--------------
src/backend/access/heap/pruneheap.c | 559 ++++++++++++++++++++----------------
1 file changed, 307 insertions(+), 252 deletions(-)