Fix bug in load elimination with multiple phis.

tests/language/vm/load_to_load_forwarding_vm_test.dart

 // Copyright (c) 2012, 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.
 // Test correctness of side effects tracking used by load to load forwarding.
 
 // VMOptions=--optimization-counter-threshold=10
 
 import "package:expect/expect.dart";
 import "dart:typed_data";
 
@@ skipping 151 matching lines @@
     a = c;
     b = xa;
     c = xb;
   }
 
   Expect.listEquals([-0.1, 0.1, 0.0,
                      0.0, -0.1, 0.1,
                      0.1, 0.0, -0.1], result);
 }
 
+
+class C {
+  C(this.box, this.parent);
+  final box;
+  final C parent;
+}
+
+testPhiForwarding5(C c) {
+  var s = 0;
+  var tmp = c;
+  var a = c.parent;
+  if (a.box + tmp.box != 1) throw "failed";
+  do {
+    s += tmp.box + a.box;
+    tmp = a;
+    a = a.parent;
+  } while (a != null);
+  return s;
+}
+
+
 class U {
   var x, y;
   U() : x = 0, y = 0;
 }
 
 testEqualPhisElimination() {
   var u = new U();
   var v = new U();
   var sum = 0;
   for (var i = 0; i < 3; i++) {
@@ skipping 71 matching lines @@
   testImmutableVMFields(fixed, true);
   testImmutableVMFields(growable, false);
   testImmutableVMFields(growable, false);
 
   final f64List = new Float64List(2);
   testPhiRepresentation(true, f64List);
   testPhiRepresentation(false, f64List);
 
   final obj = new X(new X(new X(null)));
 
+  final cs = new C(0, new C(1, new C(2, null))); 
+
   for (var i = 0; i < 20; i++) {
     Expect.listEquals([0x02010000, 0x03020100], foo(new A(0, 0)));
     Expect.listEquals([0x02010000, 0x03020100], bar(new A(0, 0), false));
     Expect.listEquals([0x04020000, 0x03020100], bar(new A(0, 0), true));
     testImmutableVMFields(fixed, true);
     testPhiRepresentation(true, f64List);
     testPhiForwarding(obj);
     testPhiForwarding2(obj);
     testPhiForwarding3();
     testPhiForwarding4();
+    Expect.equals(4, testPhiForwarding5(cs));
     testEqualPhisElimination();
   }
 
   Expect.equals(1, testImmutableVMFields([], false));
   Expect.equals(2, testImmutableVMFields([1], false));
   Expect.equals(2, testImmutableVMFields([1, 2], false));
   Expect.equals(3, testImmutableVMFields([1, 2, 3], false));
 
   final u32List = new Uint32List(3);
   u32List[0] = 0;
@@ skipping 19 matching lines @@
   for (var i = 0; i < 20; i++) {
     Expect.equals(3, testIndexedNoAlias(array));
   }
   for (var i = 0; i < 20; i++) {
     Expect.equals(5, testIndexedAliasedStore1(array, array));
   }
   for (var i = 0; i < 20; i++) {
     Expect.equals(4, testIndexedAliasedStore2(array, array, indices[1]));
   }
 }