RFC: MLIR Action, Tracing and Debugging MLIR-based Compilers

[mehdi_amini]

@scotttodd

That currently lets downstream code run code before/after each pass, without needing to modify the source code of those passes or of the pass manager.

You will be able to write instrumentations that operates on any action, that it just like pass instrumentation allows you to run your own instrumentation before/after each pass, you will be able to write an instrumentation that runs before and after a pattern application.