What Code Does Rust Pass to LLVM? Generics and Codegen Units

In the previous article, I generated MIR from Rust code and tried reading it. An if expression became a set of blocks, and we could see where values went and which block execution would enter next. But being able to read MIR doesn't mean we've reached machine code yet. LLVM apparently comes into the picture further along. What does rustc actually pass to it, and how?