Add quiet mode for mlir-opt

[jpienaar]

Summary

Allows running passes where the final case of interest is the result
(success/failure) of compilation pipeline rather than the result.
Not sure if too specialized, but for reproducer case testing I found and
even -o /dev/null felt wasteful.

[mehdi_amini]

An alias to -o /dev/null does not pull its weight IMO, unless it really disable the printer. Is this possible to have this inside MlirOptMain to guard module->print(os);?

mlir/lib/Support/MlirOptMain.cpp

LogicalResult mlir::MlirOptMain(int argc, char **argv, llvm::StringRef toolName,
                                 DialectRegistry &registry,
                                 bool preloadDialectsInContext) {
   static cl::opt<std::string> inputFilename(
       cl::Positional, cl::desc("<input file>"), cl::init("-"));
 
   static cl::opt<std::string> outputFilename("o", cl::desc("Output filename"),
                                              cl::value_desc("filename"),
                                              cl::init("-"));
+  static cl::opt<bool> quiet(
+      "quiet", cl::desc("Run passes without outputting to output file"),
+      cl::init(false));
 
   static cl::opt<bool> splitInputFile(
       "split-input-file",
       cl::desc("Split the input file into pieces and process each "
                "chunk independently"),
       cl::init(false));
 
   static cl::opt<bool> verifyDiagnostics(
if (runRepro) {
     }
     // Tokenize & parse the first line.
     SmallVector<const char *, 4> newArgv;
     newArgv.push_back(argv[0]);
     llvm::cl::TokenizeGNUCommandLine(pair.first, saver, newArgv);
     cl::ParseCommandLineOptions(newArgv.size(), &newArgv[0], helpHeader);
   }
 
-  auto output = openOutputFile(outputFilename, &errorMessage);
+  std::unique_ptr<llvm::ToolOutputFile> output;
+  if (!quiet) {
+    output = openOutputFile(outputFilename, &errorMessage);
     if (!output) {
       llvm::errs() << errorMessage << "\n";
       return failure();
     }
+  }
 
-  if (failed(MlirOptMain(output->os(), std::move(file), passPipeline, registry,
+  raw_ostream &os = quiet ? nulls() : output->os();
+  if (failed(MlirOptMain(os, std::move(file), passPipeline, registry,
                          splitInputFile, verifyDiagnostics, verifyPasses,
                          allowUnregisteredDialects, preloadDialectsInContext)))
     return failure();
 
-  // Keep the output file if the invocation of MlirOptMain was successful.
+  // Keep the output file if the invocation of MlirOptMain was successful unless
+  // in quiet mode.
+  if (!quiet)
     output->keep();
   return success();
 }