Use range information for optimizing integer boxing and fix bug in range analysis.

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
===================================================================
--- runtime/vm/flow_graph_type_propagator.cc	(revision 37585)
+++ runtime/vm/flow_graph_type_propagator.cc	(working copy)
@@ -1043,10 +1043,18 @@
 
 
 CompileType BoxIntegerInstr::ComputeType() const {
+  if (is_smi()) {
+    return CompileType::FromCid(kSmiCid);
+  }
   return CompileType::Int();
 }
 
 
+bool BoxIntegerInstr::RecomputeType() {
+  return UpdateType(ComputeType());
+}
+
+
 CompileType UnboxIntegerInstr::ComputeType() const {
   return CompileType::Int();
 }