pgsql: Trim ORDER BY/DISTINCT aggregate pathkeys in gather_grouping_pat

Trim ORDER BY/DISTINCT aggregate pathkeys in gather_grouping_paths

Similar to d8a295389, trim off any PathKeys which are for ORDER BY /
DISTINCT aggregate functions from the PathKey List for the Gather Merge
paths created by gather_grouping_paths().  These additional PathKeys are
not valid to use after grouping has taken place as these PathKeys belong
to columns which are inputs to an aggregate function and, therefore are
unavailable after aggregation.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/cf63174c-8c89-3953-cb49-48f41f74941a@gmail.com
Backpatch-through: 16, where 1349d2790 was added

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4e1ff2aadefbfbf798a4d318514ac85b2fbdc5aa

Modified Files
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src/backend/optimizer/plan/planner.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)