infinite loop on array.sml

[Stephen Weeks]

> Yeah, I get that.  It's somewhere in main_0, so it's a pain to figure out.
> I mean to ask, could the new dominator version that doesn't care about
> the graph being connected and single rooted result in a dominator tree
> that has a cycle?

Maybe there is a bug in dominator computation causing the loop, but if
it terminates, we know from the tree type

      datatype 'a t = T of 'a * 'a t vector

that the tree really is acyclic.