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/SIDebuggerInsertNops.cpp b/llvm/lib/Target/AMDGPU/SIDebuggerInsertNops.cpp
index 770a1d3..7e884ad 100644
--- a/llvm/lib/Target/AMDGPU/SIDebuggerInsertNops.cpp
+++ b/llvm/lib/Target/AMDGPU/SIDebuggerInsertNops.cpp
@@ -63,7 +63,7 @@
 bool SIDebuggerInsertNops::runOnMachineFunction(MachineFunction &MF) {
   // Skip this pass if "amdgpu-debugger-insert-nops" attribute was not
   // specified.
-  const SISubtarget &ST = MF.getSubtarget<SISubtarget>();
+  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
   if (!ST.debuggerInsertNops())
     return false;