AMDGPU: Migrate CodeGen tests to amdgpu subarch triple (8) (#208837)
Mechanically migrate the command-line target spelling on llc/opt RUN
lines in llvm/test/CodeGen/AMDGPU from -mtriple=amdgcn ... -mcpu=<gfx> to the
folded subarch triple form (e.g. -mtriple=amdgpu9.00-amd-amdhsa), dropping the
redundant -mcpu.
Co-Authored-By: Claude <noreply@anthropic.com> (Claude-Opus-4.8)