Auto merge of #156218 - cjgillot:dest-prop-range, r=Amanieu

DestinationPropagation: compute liveness as ranges instead of traveling bitsets

The current implementation of `save_as_liveness` is very slow, and consists in inserting a traveling bitset in an interval set.

As the `MaybeLiveLocals` has a gen-kill property, we can leverage it to make it faster. "Gen" is creating a new interval. "Kill" is ending this interval, ripe to save in the interval set.