Add a backward instruction iterator and use it in the liveness analysis.

vm/bit_vector_test.cc

 // 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.
 
 #include "platform/assert.h"
 #include "vm/bit_vector.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
@@ skipping 56 matching lines @@
     EXPECT_EQ(false, a->Contains(127));
     b->Add(96);
     b->Add(127);
     c->Add(127);
     a->KillAndAdd(c, b);
     EXPECT_EQ(true, a->Contains(0));
     EXPECT_EQ(true, a->Contains(32));
     EXPECT_EQ(true, a->Contains(64));
     EXPECT_EQ(true, a->Contains(96));
     EXPECT_EQ(false, a->Contains(127));
+    a->Remove(0);
+    a->Remove(32);
+    a->Remove(64);
+    a->Remove(96);
+    EXPECT_EQ(false, a->Contains(0));
+    EXPECT_EQ(false, a->Contains(32));
+    EXPECT_EQ(false, a->Contains(64));
+    EXPECT_EQ(false, a->Contains(96));
   }
 }
 
 }  // namespace dart