In the open coding of generic arithmetic it is possible to create some

In the open coding of generic arithmetic it is possible to create some
cfg node which are in the rtl-graph but turn out not to be reachable
from the entry edge of the rtl-graph (this happens when we are
creating a pcfg for a type test but the object we are testing is a
constant, for example). We need to make sure that rgraph/compress!
removes these unreachable cfg nodes, because there are some algorithms
which depened on all of node's predecessors being on a path from an
entry edge of the rtl-graph.