Graph: Optimize compute_predecessors (#76)
Graph: Optimize compute_predecessors (#76)

During SSA transformation most of the run-time was spend in
compute_predecessors. According to perf, the reason for this
was the amount of hashing required in the algorithm.
endeav0r/falcon,falconre/falcon,endeav0r/falcon,falconre/falcon,endeav0r/falcon,endeav0r/falcon,falconre/falcon