structurizer: split regions into Divergent vs Convergent, and optimize based on that.

[eddyb]

The first commit is a pretty large refactor that ended up a bit worse than I expected.
We might just want to have this instead (and call it in a few places):

impl Region {
    fn diverges(&self) -> bool {
        self.exits.is_empty()
    }
}

The second commit is just an on-the-fly OpPhi simplification that we should probably merge completely separately.

And the third commit is "clever hack" (to remove some unnecessary empty blocks, see example below) that could also probably be done without the first commit, it just didn't hit me that it was possible before the refactor.

(click to open example code & before/after graphs for the third commit)

pub fn marker() {}

pub fn shortcircuit_and(a: bool, b: bool) {
    if a && b {
        marker();
    }
}

Before

After

[khyperia]

Seems good to me! Type saftey with different types for divergent/convergent is probably kinda nice, makes you think about things instead of writing generic code for both, so I'd probably be in favor of keeping it if you like it.

[khyperia]

Just a note here that this will conflict with #563, and it might not be immediately obvious if a merge goes wonky here, so note to double check a merge goes well

[eddyb]

Closing in favor of SPIR-T (which has its own structurizer, with a different design).