i128 lowering implementation won't work for `const fn` evaluated at runtime

[nagisa]

So the code like

const fn banana(a: i128, b: i128) -> i128 {
    a / b
}

pub fn peach(a: i128, b: i128) -> i128 {
    let ban : fn(i128, i128) -> i128 = banana;
    ban(a, b)
}

Will probably ICE with how #46583 is implemented on targets with i128 lowering, however it seems like something that needn’t block the stabilisation of i128 or const fn and can be thought about when both features land.

cc #24111 #35118