[turbofan] Introduce CheckString simplified operator.

src/compiler/simplified-operator.cc

 // Copyright 2012 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-operator.h"
 
 #include "src/base/lazy-instance.h"
 #include "src/compiler/opcodes.h"
 #include "src/compiler/operator.h"
 #include "src/types.h"
@@ skipping 350 matching lines @@
   V(SpeculativeNumberSubtract)    \
   V(SpeculativeNumberDivide)      \
   V(SpeculativeNumberMultiply)    \
   V(SpeculativeNumberModulus)     \
   V(SpeculativeNumberShiftLeft)
 
 #define CHECKED_OP_LIST(V)       \
   V(CheckBounds, 2, 1)           \
   V(CheckIf, 1, 0)               \
   V(CheckNumber, 1, 1)           \
+  V(CheckString, 1, 1)           \
   V(CheckTaggedPointer, 1, 1)    \
   V(CheckTaggedSigned, 1, 1)     \
   V(CheckedInt32Add, 2, 1)       \
   V(CheckedInt32Sub, 2, 1)       \
   V(CheckedInt32Div, 2, 1)       \
   V(CheckedInt32Mod, 2, 1)       \
   V(CheckedUint32Div, 2, 1)      \
   V(CheckedUint32Mod, 2, 1)      \
   V(CheckedUint32ToInt32, 1, 1)  \
   V(CheckedFloat64ToInt32, 1, 1) \
@@ skipping 252 matching lines @@
                         Operator::kNoDeopt | Operator::kNoThrow | properties,  \
                         #Name, value_input_count, 1, control_input_count,      \
                         output_count, 1, 0, access);                           \
   }
 ACCESS_OP_LIST(ACCESS)
 #undef ACCESS
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8