MIR Optimizations Overview and Quickstart

# MIR optimizations MIR optimizations are optimizations run on the [MIR][mir] to produce better MIR before codegen. This is important for two reasons: first, it makes the final generated executable code better, and second, it means that LLVM has less work to do, so compilation is faster. Note that since MIR is generic (not [monomorphized][monomorph] yet), these optimizations are particularly effective; we can optimize the generic version, so all of the monomorphizations are cheaper! MIR optimizations run after borrow checking. We run a series of optimization passes over the MIR to improve it. Some passes are required to run on all code, some passes don't actually do optimizations but only check stuff, and some passes are only turned on in `release` mode. The [`optimized_mir`][optmir] [query] is called to produce the optimized MIR for a given [`DefId`][defid]. This query makes sure that the borrow checker has run and that some validation has occurred. Then, it [steals][steal] the MIR, optimizes it, and returns the improved MIR. ## Quickstart for adding a new optimization 1. Make a Rust source file in `tests/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 test --bless tests/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_transform/src`]. The fastest and easiest way to do this is to 1. pick a small optimization (such as [`remove_storage_markers`]) 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.

This section describes MIR optimizations, which are performed on the MIR (Mid-level Intermediate Representation) to improve code before code generation. These optimizations enhance the final executable and speed up compilation by reducing LLVM's workload. They operate on generic MIR, making them highly effective. The optimized MIR is produced by the `optimized_mir` query, which validates and optimizes the MIR. A quickstart guide is provided for adding new optimizations, including creating a test case, generating MIR dumps, and implementing the optimization based on an existing example.