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

[Shilei Tian via llvm-commits]

shiltian wrote:

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

No at the moment, since we still support r600.

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