this is for ALU cancellation. when go_die is pulled on a Comp Unit, it indicates
that the result - computed or in transit to *be* computed - is no longer required
and must be dropped on the floor, immediately.
ALU pipelines are to be set up with "cancellation" capability. this is set
from nmutil.concurrentunit.ReservationStations "maskwid" parameter, which is
set one per CompUnit. with the cancellation mask being N bits wide,
N go_die CompUnits may be connected together on the front, one go_die signal
to each cancellation mask bit.