Tag liveness: use control-flow analysis instead of backward propagation.

Tag liveness: use control-flow analysis instead of backward propagation.

This commit effectively reverts commit eeceb1d5baf3f868a64bf5fa0408f0a06cc48285,
"Use backwards propagation for liveness analyses on tags.", which changed
control flow analyses to backward propagation of liveness from final states.

The reason for rollback is that common control-flow analysis is more
general. Backward propagation is simple if tags are only used in final states.
While this is currently true, but we'll have to allow tag uses on arbitrary
arcs, and this will complicate backward propagation a lot.