[llvm] RFC: [AMDGPU] Check subtarget features for consistency (PR #86957)

[Jay Foad via llvm-commits]

jayfoad wrote:

TODO: `CodeGen/AMDGPU/unknown-processor.ll` fails because it doesn't set any subtarget features. I don't know how this should be handled. Do we really need to support `-mcpu=unknown`???

https://github.com/llvm/llvm-project/pull/86957