pgsql: Fix "constraint_exclusion = partition" logic so that it will also

Log Message:
-----------
Fix "constraint_exclusion = partition" logic so that it will also attempt
constraint exclusion on an inheritance set that is the target of an UPDATE
or DELETE query.  Per gripe from Marc Cousin.  Back-patch to 8.4 where
the feature was introduced.

Tags:
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REL8_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/nodes:
        outfuncs.c (r1.360.2.1 -> r1.360.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c?r1=1.360.2.1&r2=1.360.2.2)
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.256 -> r1.256.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c?r1=1.256&r2=1.256.2.1)
    pgsql/src/backend/optimizer/util:
        plancat.c (r1.158 -> r1.158.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/plancat.c?r1=1.158&r2=1.158.2.1)
    pgsql/src/include/nodes:
        relation.h (r1.173 -> r1.173.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.173&r2=1.173.2.1)