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/SILoadStoreOptimizer.cpp b/llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
index b35010c..4b53754 100644
--- a/llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
@@ -103,7 +103,7 @@
    };
 
 private:
-  const SISubtarget *STM = nullptr;
+  const GCNSubtarget *STM = nullptr;
   const SIInstrInfo *TII = nullptr;
   const SIRegisterInfo *TRI = nullptr;
   MachineRegisterInfo *MRI = nullptr;
@@ -939,7 +939,7 @@
   if (skipFunction(MF.getFunction()))
     return false;
 
-  STM = &MF.getSubtarget<SISubtarget>();
+  STM = &MF.getSubtarget<GCNSubtarget>();
   if (!STM->loadStoreOptEnabled())
     return false;