AMDGPU: Migrate CodeGen tests to amdgpu subarch triple (52) (#209778)
Fold the explicit -mcpu subtarget into the amdgpu subarch triple on
llc/opt RUN lines (e.g. -mtriple=amdgcn -mcpu=gfx600 ->
-mtriple=amdgpu6.00), now that these tests carry an explicit -mcpu
following the default-subtarget pinning. Codegen output is unchanged.
Co-Authored-By: Claude <noreply@anthropic.com> (Claude-Opus-4.8)