pgsql: Fix set_append_rel_pathlist() to deal intelligently with cases

Log Message:
-----------
Fix set_append_rel_pathlist() to deal intelligently with cases where
substituting a child rel's output expressions into the appendrel's restriction
clauses yields a pseudoconstant restriction.  We might be able to skip scanning
that child rel entirely (if we get constant FALSE), or generate a one-time
filter.  8.3 more or less accidentally generated plans that weren't completely
stupid in these cases, but that was only because an extra recursive level of
subquery_planner() always occurred and allowed const-simplification to happen.
8.4's ability to pull up appendrel members with non-Var outputs exposes the
fact that we need to work harder here.  Per gripe from Sergey Burladyan.

Tags:
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REL8_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        allpaths.c (r1.183 -> r1.183.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c?r1=1.183&r2=1.183.2.1)
    pgsql/src/backend/optimizer/prep:
        prepunion.c (r1.171 -> r1.171.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.171&r2=1.171.2.1)
    pgsql/src/backend/optimizer/util:
        restrictinfo.c (r1.60 -> r1.60.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/restrictinfo.c?r1=1.60&r2=1.60.2.1)
    pgsql/src/include/optimizer:
        restrictinfo.h (r1.44 -> r1.44.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/restrictinfo.h?r1=1.44&r2=1.44.2.1)