collapsible_if: comparison chain `x || y` -> `x` -> `y` can be optimized to `x && y`

I stumbled upon such a piece of code (x and y represent bool expressions):

if x || y {
    if x {
        if y {
            unimplemented!("x and y");
        } else {
            unimplemented!("x");
        }
    } else {
        unimplemented!("y");
    }
}

which can be optimized to

if x && y {
    unimplemented!("x and y");
} else if x {
    unimplemented!("x");
} else if y {
    unimplemented!("y");
}

The combination of nested if expressions

• x || y -> x -> y equals x && y
• x || y -> x equals x
• x || y -> y equals y

or

• x || y -> x -> y equals (x || y) && x && y
• x || y -> x equals (x || y) && x
• x || y -> y equals (x || y) && y

Start with the collapsible_if lint named in the issue title and compare its existing behavior with the nested boolean examples in the issue body. Determine how the lint's tests represent these condition chains; done means the intended transformations are implemented with regression coverage and their behavior is reviewed.