Set Operation._id_value before adding to control flow context.

There is a comment indicating that Operation IDs should be in
topological order, and thus the ID should be set after calling
ControlFlowContext.AddOp since it may add input ops. I believe the
comment is stale and this invariant on the IDs isn't necessary
(testing corroborates this, and also while loops cannot maintain this
since there's a cycle).

Changing this will make it easier to refactor control flow processing
in the future, since we don't have to worry about the ID not being
set.

PiperOrigin-RevId: 178457622