pgsql: Prefer actual constants to pseudo-constants in equivalence class

Prefer actual constants to pseudo-constants in equivalence class machinery.

generate_base_implied_equalities_const() should prefer plain Consts over
other em_is_const eclass members when choosing the "pivot" value that
all the other members will be equated to.  This makes it more likely that
the generated equalities will be useful in constraint-exclusion proofs.
Per report from Rushabh Lathia.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7e951ba6e1de7bb77aef4ea2d0ef7d2be860f389

Modified Files
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src/backend/optimizer/path/equivclass.c |   10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)