pgsql: Fix equivclass.c's not-quite-right strategy for handling X=X

Log Message:
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Fix equivclass.c's not-quite-right strategy for handling X=X clauses.

The original coding correctly noted that these aren't just redundancies
(they're effectively X IS NOT NULL, assuming = is strict).  However, they
got treated that way if X happened to be in a single-member EquivalenceClass
already, which could happen if there was an ORDER BY X clause, for instance.
The simplest and most reliable solution seems to be to not try to process
such clauses through the EquivalenceClass machinery; just throw them back
for traditional processing.  The amount of work that'd be needed to be
smarter than that seems out of proportion to the benefit.

Per bug #5084 from Bernt Marius Johnsen, and analysis by Andrew Gierth.

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/src/backend/optimizer:
        README (r1.49.2.1 -> r1.49.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/README?r1=1.49.2.1&r2=1.49.2.2)
    pgsql/src/backend/optimizer/path:
        equivclass.c (r1.19.2.1 -> r1.19.2.2)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/equivclass.c?r1=1.19.2.1&r2=1.19.2.2)
    pgsql/src/test/regress/expected:
        select.out (r1.20 -> r1.20.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/select.out?r1=1.20&r2=1.20.2.1)
    pgsql/src/test/regress/sql:
        select.sql (r1.15 -> r1.15.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/select.sql?r1=1.15&r2=1.15.2.1)