`VariableSlot.getLocation()` is nullable

[zcai1]

Currently, some variable slots don't have any location information (i.e., getLocation() returns null). For example, CombVariableSlot are instanciated with location equals to null in:

@Override
public CombVariableSlot createCombVariableSlot(Slot receiver, Slot declared) {
    CombVariableSlot combVariableSlot;
    Pair<Slot, Slot> pair = new Pair<>(receiver, declared);
    if (combSlotPairCache.containsKey(pair)) {
        int id = combSlotPairCache.get(pair);
        combVariableSlot = (CombVariableSlot) getSlot(id);
    } else {
        combVariableSlot = new CombVariableSlot(nextId(), null, receiver, declared);
        addToSlots(combVariableSlot);
        combSlotPairCache.put(pair, combVariableSlot.getId());
    }
    return combVariableSlot;
}

Another example is LubVariableSlot, which has a similar instanciation strategy in createMergeVariableSlot. Also, we may need special handling for the join of two branches (e.g., point to the if statement).