No, dammit, for register allocation I need to walk the blocks in *dominance*

No, dammit, for register allocation I need to walk the blocks in *dominance*
order. Since the dominance tree has changed when I fiddled with the graph, I
need to recompute it, so factor it out of the SSA pass. Code is uglier than I'd
like but at least the RET statement goes last in the generated code now.