Reimplement planner's handling of MIN/MAX aggregate optimization (again).

Instead of playing cute games with pathkeys, just build a direct
representation of the intended sub-select, and feed it through
query_planner to get a Path for the index access.  This is a bit slower
than 9.1's previous method, since we'll duplicate most of the overhead of
query_planner; but since the whole optimization only applies to rather
simple single-table queries, that probably won't be much of a problem in
practice.  The advantage is that we get to do the right thing when there's
a partial index that needs the implicit IS NOT NULL clause to be usable.
Also, although this makes planagg.c be a bit more closely tied to the
ordering of operations in grouping_planner, we can get rid of some coupling
to lower-level parts of the planner.  Per complaint from Marti Raudsepp.

src/include/optimizer/paths.h

@@ -42,14 +42,6 @@ extern void debug_print_rel(PlannerInfo *root, RelOptInfo *rel);
  * indxpath.c
  *	  routines to generate index paths
  */
-typedef enum
-{
-	/* Whether to use ScalarArrayOpExpr to build index qualifications */
-	SAOP_FORBID,				/* Do not use ScalarArrayOpExpr */
-	SAOP_ALLOW,					/* OK to use ScalarArrayOpExpr */
-	SAOP_REQUIRE				/* Require ScalarArrayOpExpr */
-} SaOpControl;
-
 extern void create_index_paths(PlannerInfo *root, RelOptInfo *rel);
 extern List *generate_bitmap_or_paths(PlannerInfo *root, RelOptInfo *rel,
 						 List *clauses, List *outer_clauses,
@@ -59,11 +51,6 @@ extern void best_inner_indexscan(PlannerInfo *root, RelOptInfo *rel,
 					 Path **cheapest_startup, Path **cheapest_total);
 extern bool relation_has_unique_index_for(PlannerInfo *root, RelOptInfo *rel,
 							  List *restrictlist);
-extern List *group_clauses_by_indexkey(IndexOptInfo *index,
-						  List *clauses, List *outer_clauses,
-						  Relids outer_relids,
-						  SaOpControl saop_control,
-						  bool *found_clause);
 extern bool eclass_matches_any_index(EquivalenceClass *ec,
 						 EquivalenceMember *em,
 						 RelOptInfo *rel);
@@ -176,9 +163,6 @@ extern List *make_pathkeys_for_sortclauses(PlannerInfo *root,
 							  List *sortclauses,
 							  List *tlist,
 							  bool canonicalize);
-extern List *make_pathkeys_for_aggregate(PlannerInfo *root,
-										 Expr *aggtarget,
-										 Oid aggsortop);
 extern void initialize_mergeclause_eclasses(PlannerInfo *root,
 											RestrictInfo *restrictinfo);
 extern void update_mergeclause_eclasses(PlannerInfo *root,