HIsStringAndBranch does an implicit SMI check if necessary.

src/code-stubs-hydrogen.cc

 // 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 867 matching lines @@
          (stub->HasSideEffects(isolate()) || !result_type->Is(Type::None())));
 
   HValue* result = NULL;
   if (stub->operation() == Token::ADD &&
       (left_type->Maybe(Type::String()) || right_type->Maybe(Type::String())) &&
       !left_type->Is(Type::String()) && !right_type->Is(Type::String())) {
     // For the generic add stub a fast case for String add is performance
     // critical.
     if (left_type->Maybe(Type::String())) {
       IfBuilder left_string(this);
-      left_string.IfNot<HIsSmiAndBranch>(left);
-      left_string.AndIf<HIsStringAndBranch>(left);
+      left_string.If<HIsStringAndBranch>(left);
       left_string.Then();
       Push(Add<HStringAdd>(left, right, STRING_ADD_CHECK_RIGHT));
       left_string.Else();
       Push(AddInstruction(BuildBinaryOperation(stub->operation(),
           left, right, left_type, right_type, result_type,
           stub->fixed_right_arg(), true)));
       left_string.End();
       result = Pop();
     } else {
       IfBuilder right_string(this);
-      right_string.IfNot<HIsSmiAndBranch>(right);
-      right_string.AndIf<HIsStringAndBranch>(right);
+      right_string.If<HIsStringAndBranch>(right);
       right_string.Then();
       Push(Add<HStringAdd>(left, right, STRING_ADD_CHECK_LEFT));
       right_string.Else();
       Push(AddInstruction(BuildBinaryOperation(stub->operation(),
           left, right, left_type, right_type, result_type,
           stub->fixed_right_arg(), true)));
       right_string.End();
       result = Pop();
     }
   } else {
@@ skipping 334 matching lines @@
   return js_function;
 }
 
 
 Handle<Code> FastNewClosureStub::GenerateCode(Isolate* isolate) {
   return DoGenerateCode(isolate, this);
 }
 
 
 } }  // namespace v8::internal