AMDGPU: Initialize WavefrontSize from TD files

[rampitec]

LGTM

lib/Target/AMDGPU/AMDGPUSubtarget.cpp

 }
 
 AMDGPUSubtarget::AMDGPUSubtarget(const Triple &TT, StringRef GPU, StringRef FS,
                                  const TargetMachine &TM)
   : AMDGPUGenSubtargetInfo(TT, GPU, FS),
     TargetTriple(TT),
     Gen(TT.getArch() == Triple::amdgcn ? SOUTHERN_ISLANDS : R600),
     IsaVersion(ISAVersion0_0_0),
-    WavefrontSize(64),
+    WavefrontSize(0),
     LocalMemorySize(0),
     LDSBankCount(0),
     MaxPrivateElementSize(0),
 
     FastFMAF32(false),
     HalfRate64Ops(false),
 
     FP32Denormals(false),

lib/Target/AMDGPU/Processors.td

 //===-- Processors.td - R600 Processor definitions ------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 class Proc<string Name, ProcessorItineraries itin, list<SubtargetFeature> Features>
 : Processor<Name, itin, Features>;
 
 // The code produced for "generic" is only useful for tests and cannot
 // reasonably be expected to execute on any particular target.
 def : ProcessorModel<"generic", NoSchedModel, [
-  FeatureGCN
+  FeatureGCN, FeatureWavefrontSize64
 ]>;
 
 //===----------------------------------------------------------------------===//
 // R600
 //===----------------------------------------------------------------------===//
 def : Proc<"r600",       R600_VLIW5_Itin,
     [FeatureR600, FeatureVertexCache, FeatureWavefrontSize64]>;