[mlir] Adopt changes in mlir-opt to standalone example

 MLIRContext was moved with commit 6bce7d8

Differential Revision: https://reviews.llvm.org/D79946

mlir/examples/standalone/standalone-opt/standalone-opt.cpp

@@ -73,8 +73,8 @@ int main(int argc, char **argv) {
   llvm::cl::ParseCommandLineOptions(argc, argv,
                                     "MLIR modular optimizer driver\n");
 
-  mlir::MLIRContext context;
   if (showDialects) {
+    mlir::MLIRContext context;
     llvm::outs() << "Registered Dialects:\n";
     for (mlir::Dialect *dialect : context.getRegisteredDialects()) {
       llvm::outs() << dialect->getNamespace() << "\n";