When replacing one value with another ensure that replacement has SSA index assigned.

runtime/vm/flow_graph_optimizer.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_FLOW_GRAPH_OPTIMIZER_H_
 #define VM_FLOW_GRAPH_OPTIMIZER_H_
 
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph.h"
 
@@ skipping 150 matching lines @@
 // A simple common subexpression elimination based
 // on the dominator tree.
 class DominatorBasedCSE : public AllStatic {
  public:
   // Return true, if the optimization changed the flow graph.
   // False, if nothing changed.
   static bool Optimize(FlowGraph* graph);
 
  private:
   static bool OptimizeRecursive(
+      FlowGraph* graph,
       BlockEntryInstr* entry,
       DirectChainedHashMap<PointerKeyValueTrait<Instruction> >* map);
 };
 
 
 // Sparse conditional constant propagation and unreachable code elimination.
 // Assumes that use lists are computed and preserves them.
 class ConstantPropagator : public FlowGraphVisitor {
  public:
   ConstantPropagator(FlowGraph* graph,
@@ skipping 48 matching lines @@
 
   // Worklists of blocks and definitions.
   GrowableArray<BlockEntryInstr*> block_worklist_;
   GrowableArray<Definition*> definition_worklist_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_