Support flattening of empty-FROM subqueries and one-row VALUES tables.

We can't handle this in the general case due to limitations of the
planner's data representations; but we can allow it in many useful cases,
by being careful to flatten only when we are pulling a single-row subquery
up into a FROM (or, equivalently, inner JOIN) node that will still have at
least one remaining relation child.  Per discussion of an example from
Kyotaro Horiguchi.

src/backend/optimizer/plan/planner.c

@@ -352,8 +352,7 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 	 * Check to see if any subqueries in the jointree can be merged into this
 	 * query.
 	 */
-	parse->jointree = (FromExpr *)
-		pull_up_subqueries(root, (Node *) parse->jointree);
+	pull_up_subqueries(root);
 
 	/*
 	 * If this is a simple UNION ALL query, flatten it into an appendrel. We