Revert r28050 due to crashes with --optimization-counter-threshold=5.

runtime/vm/flow_graph_optimizer.h

Index: runtime/vm/flow_graph_optimizer.h
===================================================================
--- runtime/vm/flow_graph_optimizer.h	(revision 28051)
+++ runtime/vm/flow_graph_optimizer.h	(working copy)
@@ -248,16 +248,8 @@
 
   static void Optimize(FlowGraph* graph);
 
-  // (1) Visit branches to optimize away unreachable blocks discovered  by range
-  // analysis.
-  // (2) Eliminate branches that have the same true- and false-target: For
-  // example, this occurs after expressions like
-  //
-  // if (a == null || b == null) {
-  //   ...
-  // }
-  //
-  // where b is known to be null.
+  // Only visit branches to optimize away unreachable blocks discovered
+  // by range analysis.
   static void OptimizeBranches(FlowGraph* graph);
 
   // Used to initialize the abstract value of definitions.
@@ -267,7 +259,6 @@
   void Analyze();
   void VisitBranches();
   void Transform();
-  void EliminateRedundantBranches();
 
   void SetReachable(BlockEntryInstr* block);
   void SetValue(Definition* definition, const Object& value);