cleanup: Unify find_use_placement approaches.

[pnkfelix]

Spawned off of: #94584 (comment)

The code for find_use_placement was at one point roughly the same as the algorithm used prior to PR #94584.

PR #94584 changed things to use a more robust algorithm. But it didn't handle the find_use_placement code.

The two could be brought into sync, as documented in a FIXME

[pnkfelix]

(I think I should be able to mentor this.)

[kckeiks]

@rustbot claim