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_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b1f7ee9218f91c755c97aefaf4494029dbf73714

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