Actually set 'modify_flags' for ARM nodes, which modify the flags.
Actually set 'modify_flags' for ARM nodes, which modify the flags.

This fixes the verifier failure of opt/compilerhang3.c.

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