Support Rvalue::Discriminant not followed by SwitchInt

[Nadrieril]

This can show up in optimized MIR, for example the optimized mir of Option::is_some(). It's not too hard to support: we can replace _x = Discriminant(_y) with

match _y {
    Variant1 => _x = <discriminant value for variant 1>,
    ...
}