Studies the subset of the optree based on C<o>, expecting it to be an C<entersub> or C<rv2cv> op (the ones you get in the C<call> and C<ref> callbacks).
If the tree is well-formed, C<*last_arg_p> will be set to the last argument of the call, C<*rv2cv_p> to the C<rv2cv> op that resolves the function name, and the C<gv> op will be returned.
Studies the subset of the optree based on C<o>, expecting it to be an C<entersub> or C<rv2cv> op (the ones you get in the C<call> and C<ref> callbacks).
If the tree is well-formed, C<*last_arg_p> will be set to the last argument of the call, C<*rv2cv_p> to the C<rv2cv> op that resolves the function name, and the C<gv> op will be returned.