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

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
diff --git a/runtime/vm/flow_graph_builder.h b/runtime/vm/flow_graph_builder.h
index ee146aa0a9ccf51022d291e898ae987a23ddd420..c6c50fe0c0007e35dededf18115bd4520191f6f2 100644
--- a/runtime/vm/flow_graph_builder.h
+++ b/runtime/vm/flow_graph_builder.h
@@ -69,14 +69,17 @@ class FlowGraphBuilder: public ValueObject {
                     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_;