Simplify query_planner's API by having it return the top-level RelOptInfo.

Formerly, query_planner returned one or possibly two Paths for the topmost
join relation, so that grouping_planner didn't see the join RelOptInfo
(at least not directly; it didn't have any hesitation about examining
cheapest_path->parent, though).  However, correct selection of the Paths
involved a significant amount of coupling between query_planner and
grouping_planner, a problem which has gotten worse over time.  It seems
best to give up on this API choice and instead return the topmost
RelOptInfo explicitly.  Then grouping_planner can pull out the Paths it
wants from the rel's path list.  In this way we can remove all knowledge
of grouping behaviors from query_planner.

The only real benefit of the old way is that in the case of an empty
FROM clause, we never made any RelOptInfos at all, just a Path.  Now
we have to gin up a dummy RelOptInfo to represent the empty FROM clause.
That's not a very big deal though.

While at it, simplify query_planner's API a bit more by having the caller
set up root->tuple_fraction and root->limit_tuples, rather than passing
those values as separate parameters.  Since query_planner no longer does
anything with either value, requiring it to fill the PlannerInfo fields
seemed pretty arbitrary.

This patch just rearranges code; it doesn't (intentionally) change any
behaviors.  Followup patches will do more interesting things.

src/backend/optimizer/util/relnode.c

@@ -676,6 +676,36 @@ subbuild_joinrel_joinlist(RelOptInfo *joinrel,
 }
 
 
+/*
+ * build_empty_join_rel
+ *		Build a dummy join relation describing an empty set of base rels.
+ *
+ * This is used for queries with empty FROM clauses, such as "SELECT 2+2" or
+ * "INSERT INTO foo VALUES(...)".  We don't try very hard to make the empty
+ * joinrel completely valid, since no real planning will be done with it ---
+ * we just need it to carry a simple Result path out of query_planner().
+ */
+RelOptInfo *
+build_empty_join_rel(PlannerInfo *root)
+{
+	RelOptInfo *joinrel;
+
+	/* The dummy join relation should be the only one ... */
+	Assert(root->join_rel_list == NIL);
+
+	joinrel = makeNode(RelOptInfo);
+	joinrel->reloptkind = RELOPT_JOINREL;
+	joinrel->relids = NULL;		/* empty set */
+	joinrel->rows = 1;			/* we produce one row for such cases */
+	joinrel->width = 0;			/* it contains no Vars */
+	joinrel->rtekind = RTE_JOIN;
+
+	root->join_rel_list = lappend(root->join_rel_list, joinrel);
+
+	return joinrel;
+}
+
+
 /*
  * find_childrel_appendrelinfo
  *		Get the AppendRelInfo associated with an appendrel child rel.