[clang] [llvm] Try to use non-volatile registers for `preserve_none` parameters (PR #88333)

[Joshua Haberman via cfe-commits]

haberman wrote:

I'm really happy to see this proposed and merged; this is how I originally envisioned it would work: https://github.com/haberman/llvm-project/commit/e8d9c75bb35ce9c802f8eac522a2c6ce003f857f

https://github.com/llvm/llvm-project/pull/88333