LogicSignal: Put all trigger matches into a single trigger stage.

LogicSignal: Put all trigger matches into a single trigger stage.

Previously each trigger match was assigned its own trigger stage,
which is problematic because devices may not support more than one
stage.

Ideally we would want the user to decide which matches go into
which stages, but that's a long way off and this change makes
things work just a little bit better for more devices.

Fixes bug #683.