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

src/IceOperand.h

 //===- subzero/src/IceOperand.h - High-level operands -----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 73 matching lines @@
       return;
     assert(Func);
     dump(Func, Func->getContext()->getStrDump());
   }
   void dump(Ostream &Str) const {
     if (BuildDefs::dump())
       dump(nullptr, Str);
   }
   /// @}
 
-  ~Operand() = default;
+  virtual ~Operand() = default;
 
 protected:
   Operand(OperandKind Kind, Type Ty) : Ty(Ty), Kind(Kind) {
     // It is undefined behavior to have a larger value in the enum
     assert(Kind <= kTarget_Max);
   }
 
   const Type Ty;
   const OperandKind Kind;
   /// Vars and NumVars are initialized by the derived class.
@@ skipping 828 matching lines @@
 private:
   const Cfg *Func;
   MetadataKind Kind;
   CfgVector<VariableTracking> Metadata;
   const static InstDefList NoDefinitions;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEOPERAND_H