Auto merge of #118175 - lqd:unify-live-loans, r=matthewjasper

Centralize live loans maintenance to fix scope differences due to liveness

As found in the recent [polonius crater run](rust-lang/rust#117593 (comment)), NLLs and the location-insensitive polonius computed different scopes on some specific CFG shapes, e.g. the following.

![image](https://github.com/rust-lang/rust/assets/247183/c3649f5e-3058-454e-854e-1a6b336bdd5e)

I had missed that liveness data was pushed from different sources than just the liveness computation: there are a few places that do this -- and some of them may be unneeded or at the very least untested, as no tests changed when I tried removing some of them.

Here, `_6` is e.g. dead on entry to `bb2[0]` during `liveness::trace`, but its regions will be marked as live later during "constraint generation" (which I plan to refactor away and put in the liveness module soon). This should cause the inflowing loans to be marked live, but they were only computed in `liveness::trace`.

Therefore, this PR moves live loan maintenance to `LivenessValues`, so that the various places pushing liveness data will all also update live loans at the same time -- except for promoteds which I don't believe need them, and their liveness handling is already interesting/peculiar.

All the regressions I saw in the initial crater run were related to this kind of shapes, and this change did fix all of them on the [next run](rust-lang/rust#117593 (comment)).

r? `@matthewjasper`

(This will conflict with #117880 but whichever lands first is fine by me, the end goal is the same for both)

Author: bors