When propagating liveness as part of the fixpoint computation we union
the live-in state of all successors into the live-out state. We then
further handle EH successors more conservatively since control can flow
to those from any point in the BB.

The all-successors include successors from two additional sources
compared to the regular successors:
1. They include the EH successors, but we already handle these
   conservatively later, so this is unnecessary.
2. They include EH successors of regular successors, but this liveness
   is already contributed from the regular successors since they will
   contain it in their live-in state at some point during the fixpoint
   computation.

Thus we can switch the first loop in liveness to only look at regular
successors.

…regular-succs