pgsql: Deduplicate choice of horizon for a relation procarray.c.

Deduplicate choice of horizon for a relation procarray.c.

5a1e1d83022 was a minimal bug fix for dc7420c2c92. To avoid future bugs of
that kind, deduplicate the choice of a relation's horizon into a new helper,
GlobalVisHorizonKindForRel().

As the code in question was only introduced in dc7420c2c92 it seems worth
backpatching this change as well, otherwise 14 will look different from all
other branches.

A different approach to this was suggested by Matthias van de Meent.

Author: Andres Freund
Discussion: https://postgr.es/m/20210621122919.2qhu3pfugxxp3cji@alap3.anarazel.de
Backpatch: 14, like 5a1e1d83022

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3d0a4636aa4c976e971c05c77e162fc70c61f40b

Modified Files
--------------
src/backend/storage/ipc/procarray.c | 98 ++++++++++++++++++++++++-------------
1 file changed, 64 insertions(+), 34 deletions(-)