pgsql: Repair logic for reordering grouping sets optimization.

Repair logic for reordering grouping sets optimization.

The logic in reorder_grouping_sets to order grouping set elements to
match a pre-specified sort ordering was defective, resulting in
unnecessary sort nodes (though the query output would still be
correct). Repair, simplifying the code a little, and add a test.

Per report from Richard Guo, though I didn't use their patch. Original
bug seems to have been my fault.

Backpatch back to 9.5 where grouping sets were introduced.

Discussion: https://postgr.es/m/CAN_9JTzyjGcUjiBHxLsgqfk7PkdLGXiM=pwM+=ph2LsWw0WO1A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/da53be23d1c5e529675e958eaee54cfed4e4dbac

Modified Files
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src/backend/optimizer/plan/planner.c | 39 ++++++++++++++----------------
src/test/regress/expected/groupingsets.out | 13 ++++++++++
src/test/regress/sql/groupingsets.sql | 3 +++
3 files changed, 34 insertions(+), 21 deletions(-)