- use mode_Iu for all nodes producing integer values (because registers are mode_Iu), this gives more possibilities for cse
- use mode_D for all nodes producing float values, this gives more possibilities for cse
- Introduce NoReg and Unknown nodes so we don't have strange projs from the start barrier
- Rewrite transform phase:
  - while transforming we now build a completely new graph on a new irg (while looking at the old graph)
  - no need to transform constants in advance anymore
  - several smaller fixes+cleanups