Optimisation: if a loop has no next edge pull one non-scc node out

[curiousdannii]

Currently nodes that are not part of a loop scc are not pulled out (ie, made a next edge) unless they are not dominated by the loop header. This means that a series of loops becomes deeply nested. As most loops have only one exit node, we can put it out as long as there isn't already a next edge. We could simply pull the first non-scc node out, or try to weigh the options somehow.

[curiousdannii]

I removed the code that did this as it was unsafe - it didn't account for descendent nodes that had merged branches. As this is only a cosmetic optimisation (reducing nested code) I won't try to implement it for now.