Add PrunedLiveness::computeSSALiveness
Straighforward API to build pruned liveness from a single SSA value.
This 3-line function allows PrunedLiveness to be used anywhere
ValueLifetimeAnalysis was used in OSSA form. Aside from simplicity and
efficiency, the difference is that this composes with other utilities
that only care about PrunedLiveBlocks, PrunedLiveness, or
PrunesLivenessBounary independent of how those data structures were generated.