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/R600MachineScheduler.cpp b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
index afded91..a1429a2 100644
--- a/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
+++ b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
@@ -346,7 +346,7 @@
   LLVM_DEBUG(dbgs() << "New Slot\n");
   assert (OccupedSlotsMask && "Slot wasn't filled");
   OccupedSlotsMask = 0;
-//  if (HwGen == R600Subtarget::NORTHERN_ISLANDS)
+//  if (HwGen == AMDGPUSubtarget::NORTHERN_ISLANDS)
 //    OccupedSlotsMask |= 16;
   InstructionsGroupCandidate.clear();
   LoadAlu();