Improve and simplify removal of unreachable code

src/hydrogen-deoptimizing-mark.h

-// Copyright 2013 the V8 project authors. All rights reserved.
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
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-//     * Redistributions in binary form must reproduce the above
-//       copyright notice, this list of conditions and the following
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-//       with the distribution.
-//     * Neither the name of Google Inc. nor the names of its
-//       contributors may be used to endorse or promote products derived
-//       from this software without specific prior written permission.
-//
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-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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-
-#ifndef V8_HYDROGEN_DEOPTIMIZING_MARK_H_
-#define V8_HYDROGEN_DEOPTIMIZING_MARK_H_
-
-#include "hydrogen.h"
-
-namespace v8 {
-namespace internal {
-
-
-// Mark all blocks that are dominated by an unconditional soft deoptimize to
-// prevent code motion across those blocks.
-class HPropagateDeoptimizingMarkPhase : public HPhase {
- public:
-  explicit HPropagateDeoptimizingMarkPhase(HGraph* graph)
-      : HPhase("H_Propagate deoptimizing mark", graph) { }
-
-  void Run();
-
- private:
-  void MarkAsDeoptimizing();
-  void NullifyUnreachableInstructions();
-
-  DISALLOW_COPY_AND_ASSIGN(HPropagateDeoptimizingMarkPhase);
-};
-
-
-} }  // namespace v8::internal
-
-#endif  // V8_HYDROGEN_DEOPTIMIZING_MARK_H_