This fixes opt/fehler183.c.

Always make pbqp coloring available

identify bitfield types based on whether they have a base_type set (maybe should introduce a real type for bitfield types?)

The algorithm now processes a block at most once,
instead of |uses| times.

This reverts commit 0a8187425d279eb16ba56f5f04465653a9dfb26a.

- use get_r/set_r instead of get_/set_* functions, so we
  can operate without current_ir_graph set
- set current_ir_graph, but only for debugging now
- fixed wrong usage of indexes for local SSA variables
- placed resorce checking at the right plaace
- removed #if 0'ed debug code and replace it by DB()
- fixed doxygen comments

Fixed typo.