Auto merge of #2341 - RalfJung:rustup, r=RalfJung

rustup; ptr atomics

Adds support for the operations added in https://github.com/rust-lang/rust/pull/96935.
I made the pointer-binops always return the provenance of the *left* argument; `@thomcc` I hope that is what you intended. I have honestly no idea if it has anything to do with what LLVM does...

I also simplified our pointer comparison code while I was at it -- now that *all* comparison operators support wide pointers, we can unify those branches.