added an assertion to get_nodes_block() that fires if this function is used on a floating node, as it would return a potential wrong result.
added an assertion to get_nodes_block() that fires
if this function is used on a floating node, as it would
return a potential wrong result.

Used get_irn_n(*, -1) in places where this is tolerable

[r6139]

MatzeB/libfirm,davidgiven/libfirm,davidgiven/libfirm,8l/libfirm,libfirm/libfirm,8l/libfirm,davidgiven/libfirm,davidgiven/libfirm,libfirm/libfirm,MatzeB/libfirm,killbug2004/libfirm,8l/libfirm,jonashaag/libfirm,8l/libfirm,davidgiven/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,killbug2004/libfirm,jonashaag/libfirm,killbug2004/libfirm,jonashaag/libfirm,killbug2004/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,MatzeB/libfirm,davidgiven/libfirm,8l/libfirm,killbug2004/libfirm,killbug2004/libfirm,MatzeB/libfirm,MatzeB/libfirm,killbug2004/libfirm,libfirm/libfirm,8l/libfirm,jonashaag/libfirm,jonashaag/libfirm,jonashaag/libfirm