[clang] [clang][SPIR-V] Add support for AMDGCN flavoured SPIRV (PR #89796)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> That's not a bad idea but I _suspect_ we'll run into a physical design issue since there doesn't seem to be a natural place to put the shared base - unless you were thinking about a place in particular? We'd probably have to relocate this to the AMDGCN side, and then duplicate the SPIRV bits/details, and at that point we could just re-use/share the reg defs.

Depending on how AMDGCN flavored this is, we could concievably just put it in the same `AMDGPU.cpp` file, since realistically both are the same "machine".

https://github.com/llvm/llvm-project/pull/89796