[turbofan] Enable typed lowering of string addition.

test/cctest/compiler/test-simplified-lowering.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <limits>
 
 #include "src/compiler/access-builder.h"
 #include "src/compiler/change-lowering.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/graph-reducer.h"
@@ skipping 1248 matching lines @@
     TestingGraph t(Type::String(), Type::String());
     IrOpcode::Value compare_eq =
         static_cast<IrOpcode::Value>(t.machine()->WordEqual()->opcode());
     IrOpcode::Value compare_lt =
         static_cast<IrOpcode::Value>(t.machine()->IntLessThan()->opcode());
     IrOpcode::Value compare_le = static_cast<IrOpcode::Value>(
         t.machine()->IntLessThanOrEqual()->opcode());
     t.CheckLoweringBinop(compare_eq, t.simplified()->StringEqual());
     t.CheckLoweringBinop(compare_lt, t.simplified()->StringLessThan());
     t.CheckLoweringBinop(compare_le, t.simplified()->StringLessThanOrEqual());
-    t.CheckLoweringBinop(IrOpcode::kCall, t.simplified()->StringAdd());
   }
 }
 
 
 void CheckChangeInsertion(IrOpcode::Value expected, MachineType from,
                           MachineType to) {
   TestingGraph t(Type::Any());
   Node* in = t.ExampleWithOutput(from);
   Node* use = t.Use(in, to);
   t.Return(use);
@@ skipping 807 matching lines @@
     Bounds phi_bounds = Bounds::Either(Bounds(d.arg1), Bounds(d.arg2), z);
     NodeProperties::SetBounds(phi, phi_bounds);
 
     Node* use = t.Use(phi, d.use);
     t.Return(use);
     t.Lower();
 
     CHECK_EQ(d.expected, OpParameter<MachineType>(phi));
   }
 }