ia32: Simplify ia32_finish_irg().
ia32: Simplify ia32_finish_irg().

The comment, stating that ia32_finish_irg_walker() starts more walks, is stale.
Therefore it is not necessary to first collect all nodes with a walker and then apply ia32_finish_irg_walker().

libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,jonashaag/libfirm,libfirm/libfirm,jonashaag/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,libfirm/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm