Ensure swap_nonoverlapping is really always untyped

[scottmcm]

This replaces #134954, which was arguably overcomplicated.

Fixes #134713

Actually using the type passed to ptr::swap_nonoverlapping for anything other than its size + align turns out to not work, so this goes back to always erasing the types down to just bytes.

(Except in const, which keeps doing the same thing as before to preserve @RalfJung's fix from #134689)

Fixes #134946

I'd previously moved the swapping to use auto-vectorization on bytes, but someone pointed out on Discord that the tail loop handling from that left a whole bunch of byte-by-byte swapping around. This goes back to manual tail handling to avoid that, then still triggers auto-vectorization on pointer-width values. (So you'll see <4 x i64> on x86-64-v3 for example.)

[rustbot]

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[RalfJung]

Could you add a Miri test for the example from #134713?

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri