[turbofan] Fix assert caused by bogus merging of out-of-scope CodeAssembler variables

src/compiler/code-assembler.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/code-assembler.h"
 
 #include <ostream>
 
 #include "src/code-factory.h"
 #include "src/compiler/graph.h"
@@ skipping 582 matching lines @@
 // properly be verified.
 class CodeAssembler::Variable::Impl : public ZoneObject {
  public:
   explicit Impl(MachineRepresentation rep) : value_(nullptr), rep_(rep) {}
   Node* value_;
   MachineRepresentation rep_;
 };
 
 CodeAssembler::Variable::Variable(CodeAssembler* assembler,
                                   MachineRepresentation rep)
-    : impl_(new (assembler->zone()) Impl(rep)) {
-  assembler->variables_.push_back(impl_);
+    : impl_(new (assembler->zone()) Impl(rep)), assembler_(assembler) {
+  assembler->variables_.insert(impl_);
 }
 
+CodeAssembler::Variable::~Variable() { assembler_->variables_.erase(impl_); }
+
 void CodeAssembler::Variable::Bind(Node* value) { impl_->value_ = value; }
 
 Node* CodeAssembler::Variable::value() const {
   DCHECK_NOT_NULL(impl_->value_);
   return impl_->value_;
 }
 
 MachineRepresentation CodeAssembler::Variable::rep() const {
   return impl_->rep_;
 }
@@ skipping 113 matching lines @@
       }
     }
   }
 
   bound_ = true;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8