(fgl) reimplement Data.Graph.Inductive.Query.Dominators

[int-e]

As pointed out at  http://www.haskell.org/pipermail/haskell-cafe/2008-April/041739.html ff., Data.Graph.Inductive.Query.Dominators.dom is buggy. Furthermore, it's slow, so instead of submitting the quick fix from that thread, I've rewritten the module from scratch using a more efficient algorithm.

The algorithm works by calculating the immediate dominators of the graph nodes first, so the patch also adds a function that returns those. It should be handy for flow graph analysis.

[int-e]

proposed patch

[int-e]

FGL is not a core package (but an extra package shipped with ghc), so the library submission guidelines don't apply.

As per Don Stewart's suggestion on libraries@… I've contacted Martin Erwig about this.

[int-e]

Martin Erwig kindly applied the patch.