MIR `MutVisitor::visit_local` cannot mutate all `Local`s

[jonas-schievink]

Specifically, when a ProjectionElem::Index is encountered, the contained Local is not passed to visit_local, requiring the MutVisitor to also implement process_projection_elem.

This is a footgun that may cause unsound MIR transforms, which is what happened to me while working on #71003. The CopyProp pass also has to explicitly implement process_projection_elem to be sound.

[jonas-schievink]

cc @rust-lang/wg-mir-opt