Replace derecursed postorder computation with one that
mimics DFS traversal.

Corrected outerinner function in loopfinder

Leave enhanced checks in place.

Change-Id: I657ba5e89c88941028d6d4c72e9f9056e30f1ce8
Reviewed-on: https://go-review.googlesource.com/40872
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>