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

src/IceAssembler.h

@@ -46,7 +46,7 @@ class Label {
 
 public:
   Label() = default;
-  ~Label() = default;
+  virtual ~Label() = default;
 
   virtual void finalCheck() const {
     // Assert if label is being destroyed with unresolved branches pending.

src/IceFixups.h

@@ -31,6 +31,7 @@ class AssemblerFixup {
 public:
   AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
+  virtual ~AssemblerFixup() = default;
   intptr_t position() const {
     assert(position_was_set_);
     return position_;

src/IceInst.h

@@ -165,7 +165,7 @@ public:
   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);

src/IceOperand.h

@@ -91,7 +91,7 @@ public:
   }
   /// @}
 
-  ~Operand() = default;
+  virtual ~Operand() = default;
 
 protected:
   Operand(OperandKind Kind, Type Ty) : Ty(Ty), Kind(Kind) {