(We have a separate get_input, and get_output callback for now).
  This should make the code faster for now and is a first step towards
  changing the interface to query register constraints on the mode_T node
  itself instead of the Proj nodes.
- Handle middleend node constraints and stuff in benode.c instead of in each
  backend
- Remove irn_class_branch we already had is_cfop in the middleend
- Fix a bunch of bugs/problems in the process

[r26320]