Change the default value for `preloadDialectsInContext` for MlirOptMain

[mehdi_amini]

Summary

This option has been deprecated for 6 months, change the default setting for now before
future removal.

While clients can set the option to true for now, they should start
updating their passes to define the right dependentDialects in
preparation of the removal of this option. See the FAQ for more info:
https://mlir.llvm.org/getting_started/Faq/

[jpienaar]

Thanks!

mlir/include/mlir/Support/MlirOptMain.h

 ///   dialects from the global registry in the MLIRContext. This option is
 ///   deprecated and will be removed soon.
 LogicalResult MlirOptMain(llvm::raw_ostream &outputStream,
                           std::unique_ptr<llvm::MemoryBuffer> buffer,
                           const PassPipelineCLParser &passPipeline,
                           DialectRegistry &registry, bool splitInputFile,
                           bool verifyDiagnostics, bool verifyPasses,
                           bool allowUnregisteredDialects,
-                          bool preloadDialectsInContext = true);
+                          bool preloadDialectsInContext = false);
 
 /// Implementation for tools like `mlir-opt`.
 /// - toolName is used for the header displayed by `--help`.
 /// - registry should contain all the dialects that can be parsed in the source.
 /// - preloadDialectsInContext will trigger the upfront loading of all
 ///   dialects from the global registry in the MLIRContext. This option is
 ///   deprecated and will be removed soon.
 LogicalResult MlirOptMain(int argc, char **argv, llvm::StringRef toolName,
                           DialectRegistry &registry,
-                          bool preloadDialectsInContext = true);
+                          bool preloadDialectsInContext = false);
 
 } // end namespace mlir
 
 #endif // MLIR_SUPPORT_MLIROPTMAIN_H