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.

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
allpaths.c (r1.183 -> r1.184)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c?r1=1.183&r2=1.184)
pgsql/src/backend/optimizer/prep:
prepunion.c (r1.171 -> r1.172)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.171&r2=1.172)
pgsql/src/backend/optimizer/util:
restrictinfo.c (r1.60 -> r1.61)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/restrictinfo.c?r1=1.60&r2=1.61)
pgsql/src/include/optimizer:
restrictinfo.h (r1.44 -> r1.45)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/restrictinfo.h?r1=1.44&r2=1.45)