Add canonicalize optimization for branches.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 16916)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -1035,6 +1035,7 @@
 
   intptr_t temp_index() const { return temp_index_; }
   void set_temp_index(intptr_t index) { temp_index_ = index; }
+  void ClearTempIndex() { temp_index_ = -1; }
 
   intptr_t ssa_temp_index() const { return ssa_temp_index_; }
   void set_ssa_temp_index(intptr_t index) {
@@ -1043,6 +1044,7 @@
     ssa_temp_index_ = index;
   }
   bool HasSSATemp() const { return ssa_temp_index_ >= 0; }
+  void ClearSSATempIndex() { ssa_temp_index_ = -1; }
 
   bool is_used() const { return (use_kind_ != kEffect); }
   void set_use_kind(UseKind kind) { use_kind_ = kind; }
@@ -1527,6 +1529,8 @@
     comparison_ = other;
   }
 
+  virtual Instruction* Canonicalize(FlowGraphOptimizer* optimizer);
+
   virtual void PrintTo(BufferFormatter* f) const;
 
  private: