- some code cleanup
- avoids the case where 2 dumpers are registered

- save ir_node* instead of irn_idx in the map (saves irn_idx_map
  lookups)
- in liveness_transfer check register class on the inputs (which is
  cheaper) and only then check the operand

Moves code from ifg dot-file dumpers around and produces .vcg graphs
now (ycomp layout is way better than xdot)