Recognize self-contradictory restriction clauses for non-table relations.

The constraint exclusion feature checks for contradictions among scan
restriction clauses, as well as contradictions between those clauses and a
table's CHECK constraints.  The first aspect of this testing can be useful
for non-table relations (such as subqueries or functions-in-FROM), but the
feature was coded with only the CHECK case in mind so we were applying it
only to plain-table RTEs.  Move the relation_excluded_by_constraints call
so that it is applied to all RTEs not just plain tables.  With the default
setting of constraint_exclusion this results in no extra work, but with
constraint_exclusion = ON we will detect optimizations that we missed
before (at the cost of more planner cycles than we expended before).

Per a gripe from Gunnlaugur Þór Briem.  Experimentation with
his example also showed we were not being very bright about the case where
constraint exclusion is proven within a subquery within UNION ALL, so tweak
the code to allow set_append_rel_pathlist to recognize such cases.

src/include/optimizer/planner.h

@@ -35,6 +35,8 @@ extern Plan *subquery_planner(PlannerGlobal *glob, Query *parse,
 				 bool hasRecursion, double tuple_fraction,
 				 PlannerInfo **subroot);
 
+extern bool is_dummy_plan(Plan *plan);
+
 extern Expr *expression_planner(Expr *expr);
 
 extern bool plan_cluster_use_sort(Oid tableOid, Oid indexOid);