Root cyclic analysis
--------------------
This analysis assigns `True` to an operation `f` if its evaluation
might result in an expression rooted by `f`.
Hence, this analysis is useful to detect simple loops.
f x = g x
g x = h x
h x = id (f x)
id x = x
Then `f`, `g`, and `h` are root-cyclic whereas `id` is not root-cyclic.