[NFC] Add const qualifier to isEmpty()

llvm/include/llvm/IR/PassManager.h

public:
   template <typename PassT>
   std::enable_if_t<std::is_same<PassT, PassManager>::value>
   addPass(PassT &&Pass) {
     for (auto &P : Pass.Passes)
       Passes.emplace_back(std::move(P));
   }
 
   /// Returns if the pass manager contains any passes.
-  bool isEmpty() { return Passes.empty(); }
+  bool isEmpty() const { return Passes.empty(); }
 
   static bool isRequired() { return true; }
 
 protected:
   using PassConceptT =
       detail::PassConcept<IRUnitT, AnalysisManagerT, ExtraArgTs...>;
 
   std::vector<std::unique_ptr<PassConceptT>> Passes;