Implement RFC 3943: MIR move elimination

[Amanieu]

This PR implements rust-lang/rfcs#3943: a MIR optimization pass that eliminates unnecessary copies. Since the new pass relies on the new MIR semantics from RFC 3943, it is gated behind the the -Z mir-move-elimination flag. Enabling this flag also disables the DestinationPropagation pass, which is completely superseded by this one.

There are 3 main parts to this optimization:

• PreciseLiveness is an analysis that calculates, at a sub-statement granularity, the points in a function where a local requires storage to be allocated. This is more fine-grained than MaybeStorageLive, and takes borrows into account.
• TailCopyToMove is a small pre-processing pass which turns copies into the return place just before a return into moves. This is necessary to allow the source of the copy to be unified with the return place.
• MoveElimination is the main pass which optimizes away moves through place unification.

The RFC text and the top-level comments for the various passes have more details on the internals of the optimization.

r? @dianqk

cc @rust-lang/wg-mir-opt

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

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

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

[rustbot]

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