AMDGPU: Cleanup subtarget handling.

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.

llvm-svn: 273652
diff --git a/llvm/lib/Target/AMDGPU/AMDILCFGStructurizer.cpp b/llvm/lib/Target/AMDGPU/AMDILCFGStructurizer.cpp
index 45f700d..16f19d8 100644
--- a/llvm/lib/Target/AMDGPU/AMDILCFGStructurizer.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDILCFGStructurizer.cpp
@@ -160,7 +160,7 @@
   bool prepare();
 
   bool runOnMachineFunction(MachineFunction &MF) override {
-    TII = static_cast<const R600InstrInfo *>(MF.getSubtarget().getInstrInfo());
+    TII = MF.getSubtarget<R600Subtarget>().getInstrInfo();
     TRI = &TII->getRegisterInfo();
     DEBUG(MF.dump(););
     OrderedBlks.clear();