Simplify liveness validation.
Simplify liveness validation.

This also fixes a possible null pointer access.

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