[clang] [flang] [llvm] [NFC][AMDGPU] Replace more direct arch comparison with isAMDGCN() (PR #131379)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

I wonder if we should just make all of these `isAMDGPU()` as well.

https://github.com/llvm/llvm-project/pull/131379