[llvm] [AMDGPU] Create dir for amdgpu specific machineverifier tests (PR #106960)

[Jay Foad via llvm-commits]

jayfoad wrote:

Please make the `amdgpu` directory name upper case, like everywhere else in `test/`.

Also it would be good to remove `REQUIRES: amdgpu-registered-target` from each individual test and into a `lit.local.cfg` but I guess that could be done in a follow up.

https://github.com/llvm/llvm-project/pull/106960