Auto merge of #87827 - eddyb:wrapperless-mem-replace, r=m-ou-se

Avoid using the `copy_nonoverlapping` wrapper through `mem::replace`.

This is a much simpler way to achieve the pre-#86003 behavior of `mem::replace` not needing dynamically-sized `memcpy`s (at least before inlining), than re-doing #81238 (which needs #86699 or something similar).

I didn't notice it until recently, but `ptr::write` already explicitly avoided using the wrapper, while `ptr::read` just called the wrapper (and was the reason for us observing any behavior change from #86003 in Rust-GPU).

<hr/>

The codegen test I've added fails without the change to `core::ptr::read` like this (ignore the `v0` mangling, I was using a worktree with it turned on by default, for this):
```llvm
       13: ; core::intrinsics::copy_nonoverlapping::<u8>
       14: ; Function Attrs: inlinehint nonlazybind uwtable
       15: define internal void `@_RINvNtCscK5tvALCJol_4core10intrinsics19copy_nonoverlappinghECsaS4X3EinRE8_25mem_replace_direct_memcpy(i8*` %src, i8* %dst, i64 %count) unnamed_addr #0 {
       16: start:
       17:  %0 = mul i64 %count, 1
       18:  call void `@llvm.memcpy.p0i8.p0i8.i64(i8*` align 1 %dst, i8* align 1 %src, i64 %0, i1 false)
not:17      !~~~~~~~~~~~~~~~~~~~~~                                                                     error: no match expected
       19:  ret void
       20: }
```
With the `core::ptr::read` change, `core::intrinsics::copy_nonoverlapping` doesn't get instantiated and the test passes.

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r? `@m-ou-se` cc `@nagisa` (codegen test) `@oli-obk` / `@RalfJung` (miri diagnostic changes)

0 files changed, 0 insertions, 0 deletions