Support else-branch for move_guard

[A4-Tacks]

Example

fn main() {
    match 92 {
        x @ 0..30 $0if x % 3 == 0 => false,
        x @ 0..30 if x % 2 == 0 => true,
        _ => false
    }
}

Before this PR

fn main() {
    match 92 {
        x @ 0..30 => if x % 3 == 0 {
            false
        },
        x @ 0..30 if x % 2 == 0 => true,
        _ => false
    }
}

After this PR

fn main() {
    match 92 {
        x @ 0..30 => if x % 3 == 0 {
            false
        } else if x % 2 == 0 {
            true
        },
        _ => false
    }
}

[ChayimFriedman2]

This will transform the code wrongly if the user is selecting the non-first arm.

Other than that, LGTM.

[A4-Tacks]

This will transform the code wrongly if the user is selecting the non-first arm.

From testing, it is expected

[ChayimFriedman2]

I meant where there is an empty selection, so all matching arms are eligible, but we'll reorder them to put the arm where the cursor is first.

[A4-Tacks]

I meant where there is an empty selection, so all matching arms are eligible

Why are we doing this? The current logic is consistent with Merge match arms