Auto merge of #72494 - lcnr:predicate-cleanup, r=nikomatsakis

Pass more `Copy` types by value.

There are a lot of locations where we pass `&T where T: Copy` by reference,
which should both be slightly less performant and less readable IMO.

This PR currently consists of three fairly self contained commits:

- passes `ty::Predicate` by value and stops depending on `AsRef<ty::Predicate>`.
- changes `<&List<_>>::into_iter` to iterate over the elements by value. This would break `List`s
  of non copy types. But as the only list constructor requires `T` to be copy anyways, I think
  the improved readability is worth this potential future restriction.
- passes `mir::PlaceElem` by value. Mir currently has quite a few copy types which are passed by reference, e.g. `Local`. As I don't have a lot of experience working with MIR, I mostly did this to get some feedback from people who use MIR more frequently
- tries to reuse `ty::Predicate` in case it did not change in some places, which should hopefully
  fix the regression caused by #72055

r? @nikomatsakis for the first commit, which continues the work of #72055 and makes adding `PredicateKind::ForAll` slightly more pleasant. Feel free to reassign though

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