[turbofan] Use Start as sentinel for frame states.

src/compiler/ast-graph-builder.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/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/js-type-feedback.h"
@@ skipping 838 matching lines @@
   UpdateStateValues(&parameters_node_, 0, parameters_count());
   UpdateStateValuesWithCache(&locals_node_, parameters_count(), locals_count());
   UpdateStateValues(&stack_node_, parameters_count() + locals_count(),
                     stack_height());
 
   const Operator* op = common()->FrameState(JS_FRAME, ast_id, combine);
 
   Node* result = graph()->NewNode(op, parameters_node_, locals_node_,
                                   stack_node_, builder()->current_context(),
                                   builder()->GetFunctionClosure(),
-                                  builder()->jsgraph()->UndefinedConstant());
+                                  builder()->graph()->start());
 
   DCHECK(IsLivenessBlockConsistent());
   if (liveness_block() != nullptr) {
     liveness_block()->Checkpoint(result);
   }
   return result;
 }
 
 
 bool AstGraphBuilder::Environment::IsLivenessAnalysisEnabled() {
@@ skipping 2927 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