pgsql: Lookup the correct ordering for parallel GIN builds

Lookup the correct ordering for parallel GIN builds

When building a tuplesort during parallel GIN builds, the function
incorrectly looked up the default B-Tree operator, not the function
associated with the GIN opclass (through GIN_COMPARE_PROC).

Fixed by using the same logic as initGinState(), and the other place
in parallel GIN builds.

This could cause two types of issues. First, a data type might not have
a B-Tree opclass, in which case the PrepareSortSupportFromOrderingOp()
fails with an ERROR. Second, a data type might have both B-Tree and GIN
opclasses, defining order/equality in different ways. This could lead to
logical corruption in the index.

Backpatch to 18, where parallel GIN builds were introduced.

Discussion: https://postgr.es/m/73a28b94-43d5-4f77-b26e-0d642f6de777@iki.fi
Reported-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/eee71a66cc860771837ed645a8dcf0ccffd735c6

Modified Files
--------------
src/backend/utils/sort/tuplesortvariants.c | 27 ++++++++++++++++++++++-----
1 file changed, 22 insertions(+), 5 deletions(-)