Make `mem::replace` simpler in codegen
Since they'd mentioned more intrinsics for simplifying stuff recently,
r? `@WaffleLapkin`
This is a continuation of me looking at foundational stuff that ends up with more instructions than it really needs. Specifically I noticed this one because `Range::next` isn't MIR-inlining, and one of the largest parts of it is a `replace::<usize>` that's a good dozen instructions instead of the two it could be.
So this means that `ptr::write` with a `Copy` type no longer generates worse IR than manually dereferencing (well, at least in LLVM -- MIR still has bonus pointer casts), and in doing so means that we're finally down to just the two essential `memcpy`s when emitting `mem::replace` for a large type, rather than the bonus-`alloca` and three `memcpy`s we emitted before this ([or the 6 we currently emit in 1.69 stable](https://rust.godbolt.org/z/67W8on6nP)). That said, LLVM does _usually_ manage to optimize the extra code away. But it's still nice for it not to have to do as much, thanks to (for example) not going through an `alloca` when `replace`ing a primitive like a `usize`.
(This is a new intrinsic, but one that's immediately lowered to existing MIR constructs, so not anything that MIRI or the codegen backends or MIR semantics needs to do work to handle.)
6db1e5e771