Refactor move analysis subpath representation

[beepster4096]

Follow up to #147055

This PR does two things:

1. Document/validate move analysis's assumptions about Subslice projections
2. Decouple move paths from ProjectionElem, using a new enum MoveSubPath instead
   • This would be needed eventually when ProjectionElem::Deref is removed

I wanted to do even more abstraction, making MovePathLookup::find return an iterator to remove the special handling of subslices in borrowck, but that regressed diagnostics and just wasn't worth the complexity.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

[rustbot]

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[RalfJung]

Is this something that the MIR validation pass could check?