pgsql: Repair planning bug introduced in 7.4: outer-join ON clauses that

Log Message:
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Repair planning bug introduced in 7.4: outer-join ON clauses that referenced
only the inner-side relation would be considered as potential equijoin clauses,
which is wrong because the condition doesn't necessarily hold above the point
of the outer join.  Per test case from Kevin Grittner (bug#1916).

Tags:
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REL7_4_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        initsplan.c (r1.91.2.2 -> r1.91.2.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/initsplan.c.diff?r1=1.91.2.2&r2=1.91.2.3)
        planmain.c (r1.78 -> r1.78.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planmain.c.diff?r1=1.78&r2=1.78.4.1)
    pgsql/src/include/optimizer:
        planmain.h (r1.75 -> r1.75.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/planmain.h.diff?r1=1.75&r2=1.75.4.1)