[PSA] `preloadDialectInContext` has been deprecated for >1y and will be removed

If you have an mlir-opt tool and you’re still depending on preloadDialectInContext, you need to revisit your pipeline. This option used with mlir-opt is hiding issues with the pipeline, and indications of missing getDependentDialects().

If you will want to still preload dialects in the future, the only workaround will likely be to fork MlirOptMain.cpp in your own project.

Just a reminder: this will likely go away this week.

This fell through as I was busy with other things and travelling, but the revision is up now: ⚙ D149039 Remove deprecated `preloadDialectInContext` flag for MlirOptMain that has been deprecated for 2 years