Add support for adding an external and a tagged pointer

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index b3536725bbfcb44e4ee5e5d3d764ef18188377b7..38f84273d0bf9522719140325a70a80b9e85eab5 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -1443,7 +1443,8 @@ Representation HAdd::RequiredInputRepresentation(int index) {
   if (index == 2) {
     Representation left_rep = left()->representation();
     if (left_rep.IsExternal()) {
-      return Representation::Integer32();
+      // Either Tagged or Integer32.
+      return Representation::None();
     }
   }
   return HArithmeticBinaryOperation::RequiredInputRepresentation(index);