using dominators in SSA optimizations

[Matthew Fluet]

> > I like the dominatorTree stuff because it is so easy to write
> > recursive functions over trees -- so maybe we need some abstraction
> > for dfs of SSA function cfg's.
> 
> Agreed.  Add val Function.dfsTree : Function.t -> Block.t Tree.t?

I added this; it works identically to dominatorsTree, except invokes
DirectedGraph.dfsTree, which does the obvious thing, giving you a dfsTree
from exactly one root.

I'll switched over the uses of dominatorsTree to dfsTree in inline,
local-flatten, loop-invariant, etc.


I'm off for the evening.