[Triton-MLIR][BACKEND] Fix the membar pass to add missing barriers caused by scf.for (#933)

1. Add missing barriers and revert the previous temporary solution
2. Extract the `run` method from membar analysis because the membar
analysis should have two phases, including construction, which doesn't
modify any IR, and modification, which adds barrier IRs. Hope this could
make the use of membar clear.

python/src/triton.cc

@@ -1251,13 +1251,12 @@ void init_triton_ir(py::module &&m) {
                                        llvm::StringRef(prefix)),
                  values);
            })
-       // Undef
-          .def("create_undef",
-               [](mlir::OpBuilder &self, mlir::Type &type) -> mlir::Value {
-               auto loc = self.getUnknownLoc();
-               return self.create<::mlir::LLVM::UndefOp>(loc, type);
-          })    
-       ;
+      // Undef
+      .def("create_undef",
+           [](mlir::OpBuilder &self, mlir::Type &type) -> mlir::Value {
+             auto loc = self.getUnknownLoc();
+             return self.create<::mlir::LLVM::UndefOp>(loc, type);
+           });
 
   py::class_<mlir::PassManager>(m, "pass_manager")
       .def(py::init<mlir::MLIRContext *>())