Eliminate phis that do not reach any non-environment uses.

runtime/vm/flow_graph_builder.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_BUILDER_H_
 #define VM_FLOW_GRAPH_BUILDER_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 51 matching lines @@
 
   void ComputeDominators(GrowableArray<BlockEntryInstr*>* preorder,
                          GrowableArray<intptr_t>* parent,
                          GrowableArray<BitVector*>* dominance_frontier);
 
   void CompressPath(intptr_t start_index,
                     intptr_t current_index,
                     GrowableArray<intptr_t>* parent,
                     GrowableArray<intptr_t>* label);
 
-  void Rename();
+  void Rename(GrowableArray<PhiInstr*>* live_phis);
   void RenameRecursive(BlockEntryInstr* block_entry,
-                       GrowableArray<Value*>* env);
+                       GrowableArray<Value*>* env,
+                       GrowableArray<PhiInstr*>* live_phis);
 
   void InsertPhis(const GrowableArray<BlockEntryInstr*>& preorder,
                   const GrowableArray<BitVector*>& assigned_vars,
                   const GrowableArray<BitVector*>& dom_frontier);
 
+  void MarkLivePhis(GrowableArray<PhiInstr*>* live_phis);
+
   const ParsedFunction& parsed_function_;
 
   const intptr_t copied_parameter_count_;
   const intptr_t non_copied_parameter_count_;
   const intptr_t stack_local_count_;  // Does not include any parameters.
 
   GrowableArray<BlockEntryInstr*> preorder_block_entries_;
   GrowableArray<BlockEntryInstr*> postorder_block_entries_;
   intptr_t context_level_;
   intptr_t last_used_try_index_;
@@ skipping 276 matching lines @@
   // Output parameters.
   TargetEntryInstr** true_successor_address_;
   TargetEntryInstr** false_successor_address_;
 
   intptr_t condition_token_pos_;
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_BUILDER_H_