Fix clang warning "has virtual functions but non-virtual destructor".

src/IceInst.h

 //===- subzero/src/IceInst.h - High-level instructions ----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 147 matching lines @@
   }
   virtual void emitIAS(const Cfg *Func) const { emit(Func); }
   virtual void dump(const Cfg *Func) const;
   virtual void dumpExtras(const Cfg *Func) const;
   void dumpDecorated(const Cfg *Func) const;
   void emitSources(const Cfg *Func) const;
   void dumpSources(const Cfg *Func) const;
   void dumpDest(const Cfg *Func) const;
   virtual bool isRedundantAssign() const { return false; }
 
-  ~Inst() = default;
+  virtual ~Inst() = default;
 
 protected:
   Inst(Cfg *Func, InstKind Kind, SizeT MaxSrcs, Variable *Dest);
   void addSource(Operand *Src) {
     assert(Src);
     assert(NumSrcs < MaxSrcs);
     Srcs[NumSrcs++] = Src;
   }
   void setLastUse(SizeT VarIndex) {
     if (VarIndex < CHAR_BIT * sizeof(LiveRangesEnded))
@@ skipping 818 matching lines @@
   static void noteHead(Ice::Inst *, Ice::Inst *) {}
   void deleteNode(Ice::Inst *) {}
 
 private:
   mutable ilist_half_node<Ice::Inst> Sentinel;
 };
 
 } // end of namespace llvm
 
 #endif // SUBZERO_SRC_ICEINST_H