[llvm] fix preserve_none in X86 backend (PR #192300)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Please update the description in the pull request.

Otherwise I think this looks okay?  Added reviewers who looked at the initial preserves_none for an opinion on whether it's okay to change the semantics like this.

https://github.com/llvm/llvm-project/pull/192300