Split AMDGPUSubtarget into amdgcn/r600 specific subclasses.
This removes most of the static_casting of the basic codegen
classes everywhere, and tries to restrict the features
visible on the wrong target.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273652 91177308-0d34-0410-b5e6-96231b3b80d8
