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/R600ISelLowering.cpp b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
index bddab43..4110e6a 100644
--- a/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
@@ -791,7 +791,7 @@
   SDValue TrigVal = DAG.getNode(TrigNode, DL, VT,
       DAG.getNode(ISD::FADD, DL, VT, FractPart,
         DAG.getConstantFP(-0.5, DL, MVT::f32)));
-  if (Gen >= R600Subtarget::R700)
+  if (Gen >= AMDGPUSubtarget::R700)
     return TrigVal;
   // On R600 hw, COS/SIN input must be between -Pi and Pi.
   return DAG.getNode(ISD::FMUL, DL, VT, TrigVal,