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

[Eli Friedman via llvm-commits]

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

Do you know how hard it would be to fix byval handling properly?  I don't really like adding bailouts like this because they make the code harder to understand.

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