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

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 40bbc902458cd70ae79d79cb73f9f3cb85447e98..bf59159164c53b4e41ae52ee2d48d76625017a1b 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -4792,8 +4792,7 @@ class HMathMinMax: public HArithmeticBinaryOperation {
   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();