(the following is the original commit message)
To optimize the algorithms coming from now, having a topological
of the nodes of the graph is benefitial. And, considering, no new
nodes will be added to the graph from this point on, we can only
compute it once.
To optimize the ordering even further, augmented graph is used.
On top of the original (and earlier added artifical) edges, the graph
used for obtaining the ordering get a few extra edges added.
`llvm::ReversePostOrderTraversal` is used to convert the graph to the
ordered node list.
Ivan Krysak