DG2Dot: refactor printing post-dominance frontiers
DG2Dot: refactor printing post-dominance frontiers

Make the code more clear and add a comment to dot file

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg