[es6] Reintroduce the instanceof operator in the backends.

src/compiler/bytecode-graph-builder.cc

 // 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.
 
 #include "src/compiler/bytecode-graph-builder.h"
 
 #include "src/compiler/bytecode-branch-analysis.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/operator-properties.h"
 #include "src/interpreter/bytecodes.h"
@@ skipping 1195 matching lines @@
 
 void BytecodeGraphBuilder::VisitTestGreaterThanOrEqual() {
   BuildCompareOp(javascript()->GreaterThanOrEqual());
 }
 
 void BytecodeGraphBuilder::VisitTestIn() {
   BuildCompareOp(javascript()->HasProperty());
 }
 
 void BytecodeGraphBuilder::VisitTestInstanceOf() {
-  DCHECK(!FLAG_harmony_instanceof);
   BuildCompareOp(javascript()->InstanceOf());
 }
 
 void BytecodeGraphBuilder::BuildCastOperator(const Operator* js_op) {
   FrameStateBeforeAndAfter states(this);
   Node* node = NewNode(js_op, environment()->LookupAccumulator());
   environment()->BindAccumulator(node, &states);
 }
 
 void BytecodeGraphBuilder::VisitToName() {
@@ skipping 463 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8