Speed up `EverInitializedPlaces`

Speed up `EverInitializedPlaces`

Currently it tracks a bit for every `Init`, but it only uses the tracked
data for locals. This means it is tracking data for projections that is
unused.
This commit shrinks the domain to only track data for `Local`s, going
from `MixedBitSet<InitIndex>` to `DenseBitSet<Local>`.
This does regress the error messages in one test:
liveness-assign-imm-local-notes.rs. The next commit will fix that.
The commit also removes some `debug` statements which probably haven't
seen use in a long time. They can be re-added easily if anyone needs
them in the future.

This commit fixes the error message regression in the previous commit by
recomputing flow information at error-reporting time.
`is_local_ever_initialized` is replaced in two ways.
- In `check_access_permissions`: by the new `first_reaching_init`
  function which picks the first init that can reach the error location
  along a path that doesn't cross `StorageDead(local)`.
- In `add_used_mut`: by a simple `contains` test.