pgsql: Split out into a separate function the code in grouping_planner()

Log Message:
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Split out into a separate function the code in grouping_planner() that
decides whether to use hashed grouping instead of sort-plus-uniq
grouping. The function needs an annoyingly large number of parameters,
but this still seems like a win for legibility, since it removes over
a hundred lines from grouping_planner (which is still too big :-().

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        planner.c (r1.182 -> r1.183)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.182&r2=1.183)