Extend GEN-UNSIGNED-DIV-BY-CONSTANT-EXPR to work for ceiling too.

A different multiplier is used so that ceiling(x/y) can be converted
to floor(m*x/2^s)+1. This doesn't work when x = 0 or y > 2^(w-1), so
those are handled separately.

TODO: More efficient method for calculating the remainder in the unsigned
ceiling transform.