[turbofan] Enable typed lowering of string addition.

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 154 matching lines @@
   V(BooleanNot)                    \
   V(BooleanToNumber)               \
   V(NumberAdd)                     \
   V(NumberSubtract)                \
   V(NumberMultiply)                \
   V(NumberDivide)                  \
   V(NumberModulus)                 \
   V(NumberToInt32)                 \
   V(NumberToUint32)                \
   V(PlainPrimitiveToNumber)        \
-  V(StringAdd)                     \
   V(ChangeTaggedToInt32)           \
   V(ChangeTaggedToUint32)          \
   V(ChangeTaggedToFloat64)         \
   V(ChangeInt32ToTagged)           \
   V(ChangeUint32ToTagged)          \
   V(ChangeFloat64ToTagged)         \
   V(ChangeBoolToBit)               \
   V(ChangeBitToBool)               \
   V(Allocate)                      \
   V(LoadField)                     \
@@ skipping 170 matching lines @@
   }
 };
 
 std::ostream& operator<<(std::ostream&, IrOpcode::Value);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_OPCODES_H_