During a code-reading exercise of linux-kernel.cat CAT file, I generated
a graph to show the to-r relations. While likely not problematic for the
model, I found it confusing that a read-write address dependency would
show as a to-r edge on the graph.

This patch therefore restricts the to-r links derived from addr to only
read-read address dependencies, so that read-write address dependencies don't
show as to-r in the graphs. This should also prevent future users of to-r from
deriving incorrect relations. Note that a read-write address dep, obviously,
still ends up in the ppo relation via the to-w relation.

I verified that a read-read address dependency still shows up as a to-r
link in the graph, as it did before.

For reference, the problematic graph was generated with the following
command:
herd7 -conf linux-kernel.cfg \
   -doshow dep -doshow to-r -doshow to-w ./foo.litmus -show all -o OUT/
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>