`i2p-p2i-opt` miscompiles rust code

[scottmcm]

Making a separate tracking issue for the core problem @nikic identified in #147265 (comment) since it's been in stable since at least May 2024.

The following rust code: https://rust.godbolt.org/z/eT9fbKvbq

#[no_mangle]
unsafe fn f(a: usize) -> u8 {
    let b = g(a) as usize;
    unsafe { *(b as *const u8) }
}

#[cold]
fn g(a: usize) -> *const u8 {
    unsafe { std::mem::transmute(a) }
}

in 1.77 compiled to

f:
        movzx   eax, byte ptr [rdi]
        ret

but starting in 1.78 becomes

f:
        ud2

The change on the rust side is #121282 (cc @saethlin), but that's just exposing the problem in this example -- the root problem could probably be repro'd on older stable too, just in less-obvious ways.

For now we mitigated this with #147541, but that's likely just a partial fix. That PR should be reverted when the issue is properly fixed.

It happens because LLVM removes the ptrtoint+inttoptr that's needed to recover exposed provenance from an otherwise-without_provenance pointer.

[RalfJung]

LLVM issue: llvm/llvm-project#33896

[apiraino]

#147541 has been backported to beta. I verified on godbolt that the regression is gone.

Would close as "fixed" but in quotes because the fix is a mitigation for a miscompilation, as per comment and this comment. I'll leave the decision to @scottmcm .

@rustbot label -i-prioritize