pgsql: Fix partition pruning with IS [NOT] NULL clauses

Fix partition pruning with IS [NOT] NULL clauses

The original code was unable to prune partitions that could not possibly
contain NULL values, when the query specified less than all columns in a
multicolumn partition key. Reorder the if-tests so that it is, and add
more commentary and regression tests.

Reported-by: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
Co-authored-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Co-authored-by: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
Co-authored-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
Reviewed-by: amul sul <sulamul(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAFjFpRc7qjLUfXLVBBC_HAnx644sjTYM=qVoT3TJ840HPbsTXw@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fb2b61a21e141843697de7982c9f6485effcd915

Modified Files
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src/backend/partitioning/partprune.c | 78 ++++++++++++++-------------
src/test/regress/expected/partition_prune.out | 41 ++++++++++++++
src/test/regress/sql/partition_prune.sql | 7 +++
3 files changed, 90 insertions(+), 36 deletions(-)