[turbofan] Proper dead code elimination as regular reducer.

src/compiler/common-operator-reducer.h

Index: src/compiler/common-operator-reducer.h
diff --git a/src/compiler/common-operator-reducer.h b/src/compiler/common-operator-reducer.h
index 5cf568e70b4258c900c2124b5c52f220788e8e7a..1456c15e957e949f1414f601b162738d5b6eb67e 100644
--- a/src/compiler/common-operator-reducer.h
+++ b/src/compiler/common-operator-reducer.h
@@ -33,6 +33,8 @@ class CommonOperatorReducer final : public AdvancedReducer {
   Reduction Reduce(Node* node) final;
 
  private:
+  Reduction ReduceBranch(Node* node);
+  Reduction ReduceMerge(Node* node);
   Reduction ReduceEffectPhi(Node* node);
   Reduction ReducePhi(Node* node);
   Reduction ReduceSelect(Node* node);