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

vm/bit_vector_test.cc

Index: vm/bit_vector_test.cc
===================================================================
--- vm/bit_vector_test.cc	(revision 9842)
+++ vm/bit_vector_test.cc	(working copy)
@@ -74,6 +74,14 @@
     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));
   }
 }