AMDGPU: Refactor Subtarget classes

Summary:
This is a follow-up to r335942.
- Merge SISubtarget into AMDGPUSubtarget and rename to GCNSubtarget
- Rename AMDGPUCommonSubtarget to AMDGPUSubtarget
- Merge R600Subtarget::Generation and GCNSubtarget::Generation into
  AMDGPUSubtarget::Generation.

Reviewers: arsenm, jvesely

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D49037

llvm-svn: 336851
diff --git a/llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h b/llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h
index f9a6e39..ca17e7c 100644
--- a/llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h
+++ b/llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h
@@ -28,7 +28,7 @@
 class ScheduleDAG;
 class SIInstrInfo;
 class SIRegisterInfo;
-class SISubtarget;
+class GCNSubtarget;
 
 class GCNHazardRecognizer final : public ScheduleHazardRecognizer {
   // This variable stores the instruction that has been emitted this cycle. It
@@ -37,7 +37,7 @@
   MachineInstr *CurrCycleInstr;
   std::list<MachineInstr*> EmittedInstrs;
   const MachineFunction &MF;
-  const SISubtarget &ST;
+  const GCNSubtarget &ST;
   const SIInstrInfo &TII;
   const SIRegisterInfo &TRI;