Don't constant fold operation if result is incompatible with it.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 4fad8502940b9adcf985a8361bffaf5bfae3f1c7..de14197f9bddcb1b004abea3fd6c9553fe5a3ee8 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -292,7 +292,12 @@ class Value : public ZoneAllocated {
         reaching_type_(NULL) { }
 
   Definition* definition() const { return definition_; }
-  void set_definition(Definition* definition) { definition_ = definition; }
+  void set_definition(Definition* definition) {
+    if ((definition_ != NULL) && (reaching_type_ != NULL)) {
+      UpdateReachingType(definition);
+    }
+    definition_ = definition;
+  }
 
   Value* previous_use() const { return previous_use_; }
   void set_previous_use(Value* previous) { previous_use_ = previous; }
@@ -329,6 +334,8 @@ class Value : public ZoneAllocated {
 
   CompileType* Type();
 
+  void UpdateReachingType(Definition* new_definition);
+
   void SetReachingType(CompileType* type) {
     reaching_type_ = type;
   }