cyclegraph: propagate RMW atomicity edges down the chain

cyclegraph: propagate RMW atomicity edges down the chain

When we add an edge between 'rmwnode' and 'tonode', we are trying to
ensure RMW atomicity. But 'rmwnode' may itself be read by another RMW
(or, in fact, a chain). So we must follow this chain down before
propagating the atomicity edge.

Also, we have the potential to run into the same problem in
addRMWEdge(), except that in that function, we assume that the RMW is
newly-explored and so does not yet have an outgoing RMW edge. This adds
an ASSERT() to check this property.