MIR move elimination [2/6]: TailCopyToMove

[Amanieu]

Depends on #163335

This PR implements a pre-pass for the MoveElimination pass from rust-lang/rfcs#3943.

TailCopyToMove turns Copy into Move just before a Return terminator. This is valid even for places that have been borrowed because the Return will invalidate borrows anyway. This is necessary to allow the source of the copy to be unified with the return place.

r? tmiasko

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Amanieu]

LLM disclosure: I actually tried getting an LLM to write this entire pass for me. It was a disaster that was unsound in new and interesting ways. That draft was promptly nuked and replaced with a properly thought out implementation. LLMs were also used to generate the boilerplate for the MIR tests, but each test was manually reviewed and adjusted before committing.