Function optimise

src/lower2/optimise2.rs:43–75

(
    instructions: Vec<Instruction>,
    max_locals: u16,
    fixed_prefix_slots: u16,
)

    Float,
    Double,
    Reference,
}

#[derive(Clone, Copy, Debug)]
struct LocalRef {
    index: u16,
    width: u16,
}

#[derive(Clone, Debug)]
struct LiveRange {
    width: u16,
    first: usize,
    last: usize,
}

#[derive(Debug)]
struct LocalLiveness {
    widths: Vec<u16>,
    uses: Vec<Option<u16>>,
    defs: Vec<Option<u16>>,
    live_in: LocalBitMatrix,
    live_out: LocalBitMatrix,
}

#[derive(Debug, Default)]
enum CompactSuccessors {
    #[default]
    None,
    One(usize),
    Two(usize, usize),
    Many(Vec<usize>),
}

impl CompactSuccessors {
    fn push(&mut self, successor: usize) {