Draft: DmdAnal: Make exnDiv behave like topDiv

... except for the purposes of dead code eliminaiton.

As far as DmdAnal is concerned, topDiv and exnDiv used to differ simply in their defaultArgDmd. NoFib suggests that nothing is lost by making the defaultArgDmd of exnDiv return topDmd instead of absDmd. (Of course the main purpose of exnDiv remains: dead code elimination.)

In the interest of simplification we should make topDiv and exnDiv as similar as possible.