If this is called mid-tick, it will be the value from before the tick started. If this is called post-tick, it will be the value from before that last tick started.
This is useful for querying the value of a signal in a testbench before some change event occurred, for example sampling a signal before a clock edge for code that was triggered on that edge.
Note that if a signal is connected to another signal, the listener may be reset.
LogicValue? get previousValue => _wire.previousValue;