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

[marbre]

Summary

MLIRContext was moved with commit 6bce7d8

[Kayjukh]

LGTM, thanks!

[marbre]

Thanks for the fast review. I would appreciate if somebody could commit on my behalf.

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

int main(int argc, char **argv) {
   // Register any pass manager command line options.
   mlir::registerPassManagerCLOptions();
   mlir::PassPipelineCLParser passPipeline("", "Compiler passes to run");
 
   // Parse pass names in main to ensure static initialization completed.
   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";
     }
     return 0;
   }
 
   // Set up the input file.