[qfcc] Clean up stray dags edges

Killed nodes can leave stray (dangling) edges that cause some confusion
in the dot graphs and may cause problems later on down the track, so
ensure there are no dangling edges.

tools/qfcc/source/dags.c

@@ -1163,6 +1163,19 @@ dag_remove_dead_nodes (dag_t *dag)
 			}
 		}
 	} while (added_root);
+
+	// clean up any stray edges that point to removed nodes
+	for (int i = 0; i < dag->num_nodes; i++) {
+		node = dag->nodes[i];
+		for (child_i = set_first (node->edges); child_i;
+			 child_i = set_next (child_i)) {
+			child = dag->nodes[child_i->element];
+			if (!set_is_member (dag->roots, child->number)
+				&& set_is_empty (child->parents)) {
+				set_remove (node->edges, child->number);
+			}
+		}
+	}
 	dag_sort_nodes (dag);
 }