Fix Smi-based MathMinMax on x64, and reenable smi mode.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4774 matching lines @@
 
   virtual Representation observed_input_representation(int index) {
     return RequiredInputRepresentation(index);
   }
 
   virtual void InferRepresentation(HInferRepresentationPhase* h_infer);
 
   virtual Representation RepresentationFromInputs() {
     Representation left_rep = left()->representation();
     Representation right_rep = right()->representation();
-    // TODO(verwaest): Initialize to Smi once lithium-codegen has been fixed.
-    Representation result = Representation::Integer32();
+    Representation result = Representation::Smi();
     result = result.generalize(left_rep);
     result = result.generalize(right_rep);
     if (result.IsTagged()) return Representation::Double();
     return result;
   }
 
   virtual bool IsCommutative() const { return true; }
 
   Operation operation() { return operation_; }
 
@@ skipping 2141 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_