[llvm] [NFCI][AMDGPU] Use X-macro to reduce boilerplate in `GCNSubtarget.h` (PR #176844)

[Shilei Tian via llvm-commits]

shiltian wrote:

> > > duplicate field names (e.g., DumpCode is set by both FeatureDumpCode and FeatureDumpCodeLower
> > 
> > 
> > Can we finally be rid of this cursed feature?
> 
> @trenouf are you aware of any remaining uses of DumpCode? I believe XGL no longer uses it to implement GetShaderDisassembly.

11 months ago, you added a dumpcode regression test. https://github.com/llvm/llvm-project/issues/116982

https://github.com/llvm/llvm-project/pull/176844