[turbofan] Add support for copy-on-write element stores.

src/compiler/simplified-lowering.cc

 // Copyright 2014 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/simplified-lowering.h"
 
 #include <limits>
 
 #include "src/address-map.h"
 #include "src/base/bits.h"
@@ skipping 2357 matching lines @@
           ProcessRemainingInputs(node, 1);
           SetOutput(node, MachineRepresentation::kTagged);
         }
         return;
       }
       case IrOpcode::kCheckMaps:
       case IrOpcode::kTransitionElementsKind: {
         VisitInputs(node);
         return SetOutput(node, MachineRepresentation::kNone);
       }
+      case IrOpcode::kEnsureWritableFastElements:
+        return VisitBinop(node, UseInfo::AnyTagged(),
+                          MachineRepresentation::kTagged);
 
       //------------------------------------------------------------------
       // Machine-level operators.
       //------------------------------------------------------------------
       case IrOpcode::kLoad: {
         // TODO(jarin) Eventually, we should get rid of all machine stores
         // from the high-level phases, then this becomes UNREACHABLE.
         LoadRepresentation rep = LoadRepresentationOf(node->op());
         ProcessInput(node, 0, UseInfo::AnyTagged());   // tagged pointer
         ProcessInput(node, 1, UseInfo::PointerInt());  // index
@@ skipping 899 matching lines @@
         isolate(), graph()->zone(), callable.descriptor(), 0, flags,
         Operator::kNoProperties);
     to_number_operator_.set(common()->Call(desc));
   }
   return to_number_operator_.get();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8