changed code placement so it can work in more environments:

changed code placement so it can work in more environments:
- added is_Block_unreachable() function which is used instead of is_Bad().
  This function used dominator information as well.
- used is_Block_dead() instead of is_Bad() in all cases
- in place_floats_early(): change the blcok of a node only if a
  non-dead block can be found
- calc_dca(): do not return dead blocks
- consumer_dom_dca(): ignore consumer in dead blocks
- use get_irn_n(*, -1) to get the initial block of a node

[r6144]