Support string add in crankshaft:

src/type-info.cc

 // Copyright 2011 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 257 matching lines @@
             return unknown;
         }
       case TRBinaryOpIC::INT32:
         if (expr->op() == Token::DIV ||
             result_type == TRBinaryOpIC::HEAP_NUMBER) {
           return TypeInfo::Double();
         }
         return TypeInfo::Integer32();
       case TRBinaryOpIC::HEAP_NUMBER:
         return TypeInfo::Double();
+      case TRBinaryOpIC::BOTH_STRING:
+        return TypeInfo::String();
       case TRBinaryOpIC::STRING:
       case TRBinaryOpIC::GENERIC:
         return unknown;
      default:
         return unknown;
     }
   }
   return unknown;
 }
 
@@ skipping 157 matching lines @@
         source_positions->Add(position);
       }
     } else {
       ASSERT(RelocInfo::IsPosition(mode));
       position = static_cast<int>(info->data());
     }
   }
 }
 
 } }  // namespace v8::internal