pgsql: Factor some code out of create_grouping_paths.

Factor some code out of create_grouping_paths.

This is preparatory refactoring to prepare the way for partition-wise
aggregate, which will reuse the new subroutines for child grouping
rels.  It also does not seem like a bad idea on general principle,
as the function was getting pretty long.

Jeevan Chalke.  The larger patch series of which this patch is a part
was reviewed and tested by Antonin Houska, Rajkumar Raghuwanshi,
Ashutosh Bapat, David Rowley, Dilip Kumar, Konstantin Knizhnik,
Pascal Legrand, and me.  Some cosmetic changes by me.

Discussion: http://postgr.es/m/CAM2+6=V64_xhstVHie0Rz=KPEQnLJMZt_e314P0jaT_oJ9MR8A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9fd8b7d632570af90a0b374816f604f59bba11ad

Modified Files
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src/backend/optimizer/plan/planner.c | 1460 ++++++++++++++++++----------------
1 file changed, 772 insertions(+), 688 deletions(-)