dart2js cps: Fix bug in bounds check elimination.

tests/compiler/dart2js/octagon_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:compiler/src/cps_ir/octagon.dart';
 import 'package:expect/expect.dart';
 
 Octagon octagon;
 SignedVariable v1, v2, v3, v4;
 
@@ skipping 215 matching lines @@
   pushConstraint(v1, v1, 2);
   pushConstraint(v1.negated, v1.negated, -2);
   pushConstraint(v2, v2, 0);
   pushConstraint(v2.negated, v2.negated, 0);
   Expect.isTrue(octagon.isSolvable, 'should be solvable');
   pushConstraint(v1, v2.negated, 0);
   Expect.isTrue(octagon.isUnsolvable, 'v1 <= v2: should be unsolvable');
 }
 
 lower_bounds_check() {
+  setup();
   SignedVariable w = octagon.makeVariable(0, 1000);
   pushConstraint(w, w, -1);
   Expect.isTrue(octagon.isUnsolvable, 'Value in range 0..1000 is not <= -1');
 }
 
 upper_bounds_check() {
+  setup();
   SignedVariable w = octagon.makeVariable(0, 1000);
   pushConstraint(w.negated, w.negated, -5000);
   Expect.isTrue(octagon.isUnsolvable, 'Value in range 0..1000 is not >= 5000');
 }
 
+diamond_graph() {
+  setup();
+  pushConstraint(v1, v2.negated, 10);
+  pushConstraint(v1, v3.negated, 1);
+  pushConstraint(v2, v3.negated, 1);
+  pushConstraint(v2, v4.negated, 2);
+  pushConstraint(v3, v2.negated, 0);
+  pushConstraint(v3, v4.negated, 100);
+  Expect.isTrue(octagon.isSolvable, 'v1 <= v4 + 3');
+  var c = pushConstraint(v4, v1.negated, -4);
+  Expect.isTrue(octagon.isUnsolvable, 'v4 <= v1 - 4 should be a contradiction');
+  popConstraint(c);
+  pushConstraint(v1.negated, v4, -4); // Check converse constraint.
+  Expect.isTrue(octagon.isUnsolvable, 'v4 <= v1 - 4 should be a contradiction');
+}
+
 void main() {
   negative_loop1();
   negative_loop2();
   negative_loop3();
   zero_loop1();
   zero_loop2();
   positive_loop1();
   positive_loop2();
   positive_and_negative_loops1();
   positive_and_negative_loops2();
   positive_and_negative_loops3();
   plus_minus1();
   constant1();
   contradict1();
   contradict2();
   lower_bounds_check();
   upper_bounds_check();
+  diamond_graph();
 }