Mark more syntax as safe; now we can run on yast2-proxy.

module, class: just containers
ivar, ivasgn: instance variables are mostly black holes because other
methods may uglify them; but they do not affect local variables

return: does affect code flow. does not invalidate known state.
  in fact it should be used for analysis, as in

  return if v.nil?
  Ops.add(v, 1)

casgn: how to treat constants? probably should not care as they are not
  used in arguments to zombies