Simplify box_overlap computations.

Given the assumption that a box's high coordinates are not less than its
low coordinates, the tests in box_ov() are overly complicated and can be
reduced to about half as much work.  Since many other functions in
geo_ops.c rely on that assumption, there doesn't seem to be a good reason
not to use it here.

Per discussion of Alexander Korotkov's GiST fix, which was already using
the simplified logic (in a non-fuzzy form, but the equivalence holds just
as well for fuzzy).

src/backend/utils/adt/geo_ops.c

@@ -558,15 +558,10 @@ box_overlap(PG_FUNCTION_ARGS)
 static bool
 box_ov(BOX *box1, BOX *box2)
 {
-	return ((FPge(box1->high.x, box2->high.x) &&
-			 FPle(box1->low.x, box2->high.x)) ||
-			(FPge(box2->high.x, box1->high.x) &&
-			 FPle(box2->low.x, box1->high.x)))
-		&&
-		((FPge(box1->high.y, box2->high.y) &&
-		  FPle(box1->low.y, box2->high.y)) ||
-		 (FPge(box2->high.y, box1->high.y) &&
-		  FPle(box2->low.y, box1->high.y)));
+	return (FPle(box1->low.x, box2->high.x) &&
+			FPle(box2->low.x, box1->high.x) &&
+			FPle(box1->low.y, box2->high.y) &&
+			FPle(box2->low.y, box1->high.y));
 }
 
 /*		box_left		-		is box1 strictly left of box2?