Prune unused stateless nodes from function bodies.

Previously, all nodes in a TensorFlow function would be executed
unconditionally, which led to surprising performance issues (such as
executing a expensive image summary op that was created but unused in
a preprocessing function). We can prune nodes that are not
reverse-reachable from the return values of a function if they are
stateless and are not reverse-reachable from a stateful node.

PiperOrigin-RevId: 179430810