[interpreter] Remove BytecodeArrayBuilder::Illegal().

src/interpreter/bytecode-peephole-optimizer.h

 // Copyright 2015 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_INTERPRETER_BYTECODE_PEEPHOLE_OPTIMIZER_H_
 #define V8_INTERPRETER_BYTECODE_PEEPHOLE_OPTIMIZER_H_
 
 #include "src/interpreter/bytecode-pipeline.h"
 
 namespace v8 {
@@ skipping 27 matching lines @@
   void SetLast(const BytecodeNode* const node);
 
   bool LastBytecodePutsNameInAccumulator() const;
 
   Handle<Object> GetConstantForIndexOperand(const BytecodeNode* const node,
                                             int index) const;
 
   ConstantArrayBuilder* constant_array_builder_;
   BytecodePipelineStage* next_stage_;
   BytecodeNode last_;
-  bool last_is_valid_;
   bool last_is_discardable_;
 
   DISALLOW_COPY_AND_ASSIGN(BytecodePeepholeOptimizer);
 };
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_INTERPRETER_BYTECODE_PEEPHOLE_OPTIMIZER_H_