[turbofan] Introduce CheckString simplified operator.

src/compiler/opcodes.h

 // Copyright 2013 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.
 
 #ifndef V8_COMPILER_OPCODES_H_
 #define V8_COMPILER_OPCODES_H_
 
 #include <iosfwd>
 
 // Opcodes for control operators.
@@ skipping 249 matching lines @@
   V(NumberTan)                        \
   V(NumberTanh)                       \
   V(NumberTrunc)                      \
   V(NumberToInt32)                    \
   V(NumberToUint32)                   \
   V(NumberSilenceNaN)                 \
   V(StringFromCharCode)               \
   V(CheckBounds)                      \
   V(CheckIf)                          \
   V(CheckNumber)                      \
+  V(CheckString)                      \
   V(CheckTaggedPointer)               \
   V(CheckTaggedSigned)                \
   V(CheckFloat64Hole)                 \
   V(CheckTaggedHole)                  \
   V(Allocate)                         \
   V(LoadField)                        \
   V(LoadBuffer)                       \
   V(LoadElement)                      \
   V(StoreField)                       \
   V(StoreBuffer)                      \
@@ skipping 450 matching lines @@
   }
 };
 
 std::ostream& operator<<(std::ostream&, IrOpcode::Value);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_OPCODES_H_