annotate-loops
Adds a :loops field to nodes that represent a code path that
might be visited more than once because of a recur.
The field is a set of loop-ids representing the loops that might
recur into that path
Note that because (recur expr) is equivalent to (let [e expr] (recur e))
the node corresponting to expr will have the same :loops field
as the nodes in the same code path of the recur