- This is sufficient, because every endless loop must have one kept block.
- This also prevents that the if-conversion enters endless loops at strange points.
  This lead to exchanging a block, which is currently being recursed, so when later returning to the exchanged block (which now is an Id-node) in the walker stuff blew up.

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.