[llvm] [Clang][inlineasm] Add special support for "rm" output constraints (PR #92040)

[Kees Cook via llvm-commits]

https://github.com/kees approved this pull request.

This looks good to me: the comments are updated where needed; the register preference is limited to x86; "m" is still available on fallback; tests include the different allocators both with and without register pressure.

https://github.com/llvm/llvm-project/pull/92040