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

[Matt Arsenault via llvm-commits]

arsenm 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`???

We need to support the none case for library builds, but I've been debating rejecting codegenning it. It's a nonstop source of bug reports where the default of "almost Tahiti" fails to select flat pointer load/store 


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