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Add quickstart for adding a new optimization (#1094)

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@ -26,6 +26,55 @@ optimizes it, and returns the improved MIR.
[defid]: https://rustc-dev-guide.rust-lang.org/appendix/glossary.html#def-id
[steal]: https://rustc-dev-guide.rust-lang.org/mir/passes.html?highlight=steal#stealing
## Quickstart for adding a new optimization
1. Make a Rust source file in `src/test/mir-opt` that shows the code you want to
optimize. This should be kept simple, so avoid `println!` or other formatting
code if it's not necessary for the optimization. The reason for this is that
`println!`, `format!`, etc. generate a lot of MIR that can make it harder to
understand what the optimization does to the test.
2. Run `./x.py test --bless src/test/mir-opt/<your-test>.rs` to generate a MIR
dump. Read [this README][mir-opt-test-readme] for instructions on how to dump
things.
3. Commit the current working directory state. The reason you should commit the
test output before you implement the optimization is so that you (and your
reviewers) can see a before/after diff of what the optimization changed.
4. Implement a new optimization in [`compiler/rustc_mir/src/transform`].
The fastest and easiest way to do this is to
1. pick a small optimization (such as [`no_landing_pads`]) and copy it
to a new file,
2. add your optimization to one of the lists in the
[`run_optimization_passes()`] function,
3. and then start modifying the copied optimization.
5. Rerun `./x.py test --bless src/test/mir-opt/<your-test>.rs` to regenerate the
MIR dumps. Look at the diffs to see if they are what you expect.
6. Run `./x.py test src/test/ui` to see if your optimization broke anything.
7. If there are issues with your optimization, experiment with it a bit and
repeat steps 5 and 6.
8. Commit and open a PR. You can do this at any point, even if things aren't
working yet, so that you can ask for feedback on the PR. Open a "WIP" PR
(just prefix your PR title with `[WIP]` or otherwise note that it is a
work in progress) in that case.
Make sure to commit the blessed test output as well! It's necessary for CI to
pass and it's very helpful to reviewers.
If you have any questions along the way, feel free to ask in
`#t-compiler/wg-mir-opt` on Zulip.
[mir-opt-test-readme]: https://github.com/rust-lang/rust/blob/master/src/test/mir-opt/README.md
[`compiler/rustc_mir/src/transform`]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_mir/src/transform
[`no_landing_pads`]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir/src/transform/no_landing_pads.rs
[`run_optimization_passes()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/fn.run_optimization_passes.html
## Defining optimization passes
The list of passes run and the order in which they are run is defined by the