[turbofan] Introduce CheckString simplified 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/address-map.h"
 #include "src/base/bits.h"
@@ skipping 1972 matching lines @@
             // that at some point, and maybe even a dedicated truncation.
             VisitUnop(node, UseInfo::AnyTagged(),
                       MachineRepresentation::kTagged);
           }
           if (lower()) DeferReplacement(node, node->InputAt(0));
         } else {
           VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
         }
         return;
       }
+      case IrOpcode::kCheckString: {
+        if (InputIs(node, Type::String())) {
+          VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
+          if (lower()) DeferReplacement(node, node->InputAt(0));
+        } else {
+          VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
+        }
+        return;
+      }
       case IrOpcode::kCheckTaggedPointer: {
         if (InputCannotBe(node, Type::SignedSmall())) {
           VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
           if (lower()) DeferReplacement(node, node->InputAt(0));
         } else {
           VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
         }
         return;
       }
       case IrOpcode::kCheckTaggedSigned: {
@@ skipping 1501 matching lines @@
         isolate(), graph()->zone(), callable.descriptor(), 0, flags,
         Operator::kNoProperties);
     to_number_operator_.set(common()->Call(desc));
   }
   return to_number_operator_.get();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8