pgsql: Avoid making a separate pass over the query to check for partial

Avoid making a separate pass over the query to check for partializability.

It's rather silly to make a separate pass over the tlist + HAVING qual,
and a separate set of visits to the syscache, when get_agg_clause_costs
already has all the required information in hand.  This nets out as less
code as well as fewer cycles.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f1993038a4f0ce5fbeb7b562b2acd571bf6b567b

Modified Files
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src/backend/optimizer/plan/planner.c |  29 +++++----
src/backend/optimizer/util/clauses.c | 118 ++++++++++-------------------------
src/include/nodes/relation.h         |   5 +-
src/include/optimizer/clauses.h      |  20 ------
4 files changed, 54 insertions(+), 118 deletions(-)