try to write heuristics to decide dynamically if `use_ite_for_select` is worth it or not for a given request

[redianthus]

see #611

I wouldn't be surprised if something quite simple was enough to get almost the best result in each case, for instance something like: if the expression is small (its size + number of symbols) then it is better to use ite, otherwise it is better to use proper branching.