Implement IsNullCheck for CheckClassInstr. If input is nullable expected type, then check for null …

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 25902)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -6330,10 +6330,8 @@
 
   virtual void PrintOperandsTo(BufferFormatter* f) const;
 
-  void set_null_check(bool flag) { null_check_ = flag; }
+  bool IsNullCheck() const;
 
-  bool null_check() const { return null_check_; }
-
   virtual bool AllowsCSE() const { return true; }
   virtual EffectSet Effects() const { return EffectSet::None(); }
   virtual EffectSet Dependencies() const;
@@ -6344,8 +6342,6 @@
  private:
   const ICData& unary_checks_;
 
-  bool null_check_;
-
   DISALLOW_COPY_AND_ASSIGN(CheckClassInstr);
 };