pgsql: Fix bogus logic for combining range-partitioned columns duringp

Fix bogus logic for combining range-partitioned columns during pruning.

gen_prune_steps_from_opexps's notion of how to do this was overly
complicated and underly correct.

Per discussion of a report from Alan Jackson (though this fixes only one
aspect of that problem).  Back-patch to v11 where this code came in.

Amit Langote

Discussion: https://postgr.es/m/FAD28A83-AC73-489E-A058-2681FA31D648@tvsquared.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/51948c4e1fdef88ba9b953bd7b58d19a348732be

Modified Files
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src/backend/partitioning/partprune.c          | 46 ++++++---------------------
src/test/regress/expected/partition_prune.out | 27 ++++++++++++++++
src/test/regress/sql/partition_prune.sql      | 19 +++++++++++
3 files changed, 55 insertions(+), 37 deletions(-)