umin_seq select modelling requires that one of the hands of the select must be a constant.
After we remove pointer base, in all but most trivial cases,
there will likely be some other offset between the base and the selected-between pointers,
and if both of them happen to have some variable offset, we fail,
even if they have *the same* variable offset.
Instead, we should try to factor it out, perhaps that leaves us with a constant in one hand.