Subzero: Fix some -Winconsistent-missing-override that clang 3.6 warns about.

src/IceInst.h

Index: src/IceInst.h
diff --git a/src/IceInst.h b/src/IceInst.h
index 8dd77eca231f8d9bd2309c4f054636e5a5896653..2d6d0548ef26c5d21fe1282c7dfadcf25f7d9956 100644
--- a/src/IceInst.h
+++ b/src/IceInst.h
@@ -371,7 +371,7 @@ public:
   Operand *getArg(SizeT I) const { return getSrc(I + 1); }
   SizeT getNumArgs() const { return getSrcSize() - 1; }
   bool isTailcall() const { return HasTailCall; }
-  void dump(const Cfg *Func) const;
+  void dump(const Cfg *Func) const override;
   static bool classof(const Inst *Inst) { return Inst->getKind() == Call; }
   Type getReturnType() const;