A bus skew constraint is set on a safely timed crossing that is either a
synchronous clock-domain-crossing path or an intra-clock domain path. The set_bus_skew constraint should only be used on asynchronous
paths. Although set_bus_skew is generally recommended to
be used in conjunction with set_max_delay -datapath_only
to control the relative placement between the source and destination registers, the
constraint could also be used on a synchronous path when it is covered by a false path
exception.