`i2p-p2i-opt` miscompiles rust code

Making a separate tracking issue for the core problem @nikic identified in https://github.com/rust-lang/rust/issues/147265#issuecomment-3360729310 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 https://github.com/rust-lang/rust/pull/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 https://github.com/rust-lang/rust/pull/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.

Start by reproducing the Rust code and assembly difference in the linked Godbolt example, then read Rust issues 147265 and 147538 plus pull requests 121282 and 147541. Trace the LLVM handling of the ptrtoint and inttoptr sequence for exposed provenance; done means the miscompilation is fixed and the mitigation in pull request 147541 can be reverted.