[turbofan] Introduce ObjectIsString operator.

src/compiler/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 "src/compiler/simplified-lowering.h"
 
 #include <limits>
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 1174 matching lines @@
         ProcessInput(node, 2,
                      TruncatingUseInfoFromRepresentation(
                          access.machine_type.representation()));  // value
         ProcessRemainingInputs(node, 3);
         SetOutput(node, MachineRepresentation::kNone);
         break;
       }
       case IrOpcode::kObjectIsNumber:
       case IrOpcode::kObjectIsReceiver:
       case IrOpcode::kObjectIsSmi:
+      case IrOpcode::kObjectIsString:
       case IrOpcode::kObjectIsUndetectable: {
         ProcessInput(node, 0, UseInfo::AnyTagged());
         SetOutput(node, MachineRepresentation::kBit);
         break;
       }
 
       //------------------------------------------------------------------
       // Machine-level operators.
       //------------------------------------------------------------------
       case IrOpcode::kLoad: {
@@ skipping 972 matching lines @@
   if (!rhs_type->Is(type_cache_.kZeroToThirtyOne)) {
     node->ReplaceInput(1, graph()->NewNode(machine()->Word32And(), rhs,
                                            jsgraph()->Int32Constant(0x1f)));
   }
   NodeProperties::ChangeOp(node, op);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8