match lowering: expand or-candidates mixed with candidates above

[Nadrieril]

This PR tweaks match lowering of or-patterns. Consider this:

match (x, y) {
    (1, true) => 1,
    (2, false) => 2,
    (1 | 2, true | false) => 3,
    (3 | 4, true | false) => 4,
    _ => 5,
}

One might hope that this can be compiled to a single SwitchInt on x followed by some boolean checks. Before this PR, we compile this to 3 SwitchInts on x, because an arm that contains more than one or-pattern was compiled on its own. This PR groups branch 3 with the two branches above, getting us down to 2 SwitchInts on x.

We can't in general expand or-patterns freely, because this interacts poorly with another optimization we do: or-pattern simplification. When an or-pattern doesn't involve bindings, we branch the success paths of all its alternatives to the same block. The drawback is that in a case like:

match (1, true) {
    (1 | 2, false) => unreachable!(),
    (2, _) => unreachable!(),
    _ => {}
}

if we used a single SwitchInt, by the time we test false we don't know whether we came from the 1 case or the 2 case, so we don't know where to go if false doesn't match.

Hence the limitation: we can process or-pattern alternatives alongside candidates that precede it, but not candidates that follow it. (Unless the or-pattern is the only remaining match pair of its candidate, in which case we can process it alongside whatever).

This PR allows the processing of or-pattern alternatives alongside candidates that precede it. One benefit is that we now process or-patterns in a single place in mod.rs.

r? @matthewjasper

[matthewjasper]

@bors r+

[bors]

📌 Commit 7b764be has been approved by matthewjasper

It is now in the queue for this repository.