[turbofan] Introduce a dedicated StringCharAt 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>
 
 #include "src/globals.h"
@@ skipping 281 matching lines @@
   V(NumberToInt32)                     \
   V(NumberToUint32)                    \
   V(NumberToUint8Clamped)              \
   V(NumberSilenceNaN)
 
 #define SIMPLIFIED_OTHER_OP_LIST(V) \
   V(PlainPrimitiveToNumber)         \
   V(PlainPrimitiveToWord32)         \
   V(PlainPrimitiveToFloat64)        \
   V(BooleanNot)                     \
+  V(StringCharAt)                   \
   V(StringCharCodeAt)               \
   V(StringFromCharCode)             \
   V(StringFromCodePoint)            \
   V(CheckBounds)                    \
   V(CheckIf)                        \
   V(CheckMaps)                      \
   V(CheckNumber)                    \
   V(CheckString)                    \
   V(CheckSmi)                       \
   V(CheckHeapObject)                \
@@ skipping 488 matching lines @@
   }
 };
 
 V8_EXPORT_PRIVATE std::ostream& operator<<(std::ostream&, IrOpcode::Value);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_OPCODES_H_