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

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 07491af2ef982fbef7f61269672084cc4676966c..8245b46edf4d2d5b1c37c44e6733f749cccde7cc 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -2216,6 +2216,7 @@ class JoinEntryInstr : public BlockEntryInstr {
   virtual void PrepareEntry(FlowGraphCompiler* compiler);
 
   void InsertPhi(intptr_t var_index, intptr_t var_count);
+  void RemoveDeadPhis();
 
   intptr_t phi_count() const { return phi_count_; }
 
@@ -2362,7 +2363,8 @@ class BindInstr : public Definition {
 
 class PhiInstr : public Definition {
  public:
-  explicit PhiInstr(intptr_t num_inputs) : inputs_(num_inputs) {
+  explicit PhiInstr(intptr_t num_inputs)
+    : inputs_(num_inputs), is_alive_(false) {
     for (intptr_t i = 0; i < num_inputs; ++i) {
       inputs_.Add(NULL);
     }
@@ -2381,10 +2383,15 @@ class PhiInstr : public Definition {
 
   virtual bool CanDeoptimize() const { return false; }
 
+  // Phi is alive if it reaches a non-environment use.
+  bool is_alive() const { return is_alive_; }
+  void mark_alive() { is_alive_ = true; }
+
   DECLARE_INSTRUCTION(Phi)
 
  private:
   GrowableArray<Value*> inputs_;
+  bool is_alive_;
 
   DISALLOW_COPY_AND_ASSIGN(PhiInstr);
 };
@@ -2690,6 +2697,10 @@ class Environment : public ZoneAllocated {
     return values_;
   }
 
+  GrowableArray<Value*>* values_ptr() {
+    return &values_;
+  }
+
   void InitializeLocations() {
     location_count_ = values_.length();
     if (location_count_ > 0) {