Display of CFGs and DomTrees

Would be fabolous to add some pretty printing for the CFG and dominator trees.

Keno apparently uses this snippet for DomTrees. We could use dot for the CFG like opt does it.

using AbstractTrees
using Core.Compiler: DomTree

AbstractTrees.treekind(d::DomTree) = AbstractTrees.IndexedTree()
AbstractTrees.childindices(d::DomTree, i::Int) = d[i].children
AbstractTrees.childindices(::DomTree, ::DomTree) = (1,)
AbstractTrees.parentlinks(::DomTree) = AbstractTrees.StoredParents()
AbstractTrees.printnode(io::IO, i::Int, d::DomTree) = print(io, i)
Base.getindex(d::DomTree, i) = d.nodes[i]
Base.show(io::IO, d::DomTree) = print_tree(io, 1; roottree = d)