match lowering: pre-simplify or-patterns too
This is the final part of my work to simplify match pairs early: now we do it for or-patterns too. This makes it possible to collect fake borrows separately from the main match lowering algorithm. That'll enable more simplifications of or-pattern handling.
Note: I was tempted to have `Candidate` contain a `FlatPat`, but there are so many places that use `candidate.match_pairs` etc directly that I chose not to.
r? `@matthewjasper`
b2c933a1b9