[turbofan] Introduce CheckString simplified operator.

src/compiler/verifier.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/verifier.h"
 
 #include <algorithm>
 #include <deque>
 #include <queue>
 #include <sstream>
@@ skipping 947 matching lines @@
 
     case IrOpcode::kCheckBounds:
       CheckValueInputIs(node, 0, Type::Any());
       CheckValueInputIs(node, 1, Type::Unsigned31());
       CheckUpperIs(node, Type::Unsigned31());
       break;
     case IrOpcode::kCheckNumber:
       CheckValueInputIs(node, 0, Type::Any());
       CheckUpperIs(node, Type::Number());
       break;
+    case IrOpcode::kCheckString:
+      CheckValueInputIs(node, 0, Type::Any());
+      CheckUpperIs(node, Type::String());
+      break;
     case IrOpcode::kCheckIf:
       CheckValueInputIs(node, 0, Type::Boolean());
       CheckNotTyped(node);
       break;
     case IrOpcode::kCheckTaggedSigned:
       CheckValueInputIs(node, 0, Type::Any());
       CheckUpperIs(node, Type::TaggedSigned());
       break;
     case IrOpcode::kCheckTaggedPointer:
       CheckValueInputIs(node, 0, Type::Any());
@@ skipping 546 matching lines @@
          replacement->op()->EffectOutputCount() > 0);
   DCHECK(!NodeProperties::IsFrameStateEdge(edge) ||
          replacement->opcode() == IrOpcode::kFrameState);
 }
 
 #endif  // DEBUG
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8