Auto merge of #141667 - lqd:lazy-maybe-init, r=matthewjasper

Add fast path for maybe-initializedness in liveness

r? `@matthewjasper`

Correct me if I'm wrong Matthew, but my understanding is that
1. `MaybeInitializedPlaces` is currently eagerly computed, in `do_mir_borrowck`
2. but this data is only used in liveness
3. and `liveness::trace` actually only uses it for drop-liveness

This PR moves the computation to `liveness::trace` which looks to be its only use-site. We also add a fast path there, so that it's only computed by drop-liveness.

This is interesting because 1) liveness is only computed for relevant live locals, 2) drop-liveness is only computed for relevant live locals with >0 drop points; 0 is the common case from our benchmarks, as far as I can tell, so even just computing the entire data lazily helps.

It seems possible to also reduce the ___domain here, and speed up the analysis for the cases where it has to be computed -- so I've left a fixme for that, and may look into it soon.

(I've come upon this while doing implementation work for polonius, so don't be too enamored with possible wins: the goal is to reduce the eventual polonius overhead and make it more palatable 😓)

Author: bors