Fix beabi call sorting

Difference of idx for Calls with no order relation is stable,
but may lead to circular dependencies. Fixed by looking at
the heights first.

Fixes backend/transform_loop.c (with -O0)

[r28436]

ir/be/beabi.c

@@ -979,6 +979,7 @@ static int cmp_call_dependency(const void *c1, const void *c2)
 {
 	ir_node *n1 = *(ir_node **) c1;
 	ir_node *n2 = *(ir_node **) c2;
+	unsigned h1, h2;
 
 	/*
 		Classical qsort() comparison function behavior:
@@ -993,7 +994,16 @@ static int cmp_call_dependency(const void *c1, const void *c2)
 		return 1;
 
 	/* The nodes have no depth order, but we need a total order because qsort()
-	 * is not stable. */
+	 * is not stable.
+	 *
+	 * Additionally, we need to respect transitive dependencies. Consider a
+	 * Call a depending on Call b and an independent Call c.
+	 * We MUST NOT order c > a and b > c. */
+	h1 = get_irn_height(ir_heights, n1);
+	h2 = get_irn_height(ir_heights, n2);
+	if (h1 < h2) return -1;
+	if (h1 > h2) return  1;
+	/* Same height, so use a random (but stable) order */
 	return get_irn_idx(n1) - get_irn_idx(n2);
 }