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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master

Details
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http://git.postgresql.org/pg/commitdiff/17804fa71b4a4e7a099f780616a7b53ea591774d

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