[AMDGPU] Grab MCSubtargetInfo from TargetMachine instead of constructing it

[arsenm]

LGTM

llvm/trunk/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp

 StringRef AMDGPUAsmPrinter::getPassName() const {
   return "AMDGPU Assembly Printer";
 }
 
 void AMDGPUAsmPrinter::EmitStartOfAsmFile(Module &M) {
   if (TM.getTargetTriple().getOS() != Triple::AMDHSA)
     return;
 
-  // Need to construct an MCSubtargetInfo here in case we have no functions
-  // in the module.
-  std::unique_ptr<MCSubtargetInfo> STI(TM.getTarget().createMCSubtargetInfo(
-        TM.getTargetTriple().str(), TM.getTargetCPU(),
-        TM.getTargetFeatureString()));
-
   AMDGPUTargetStreamer *TS =
       static_cast<AMDGPUTargetStreamer *>(OutStreamer->getTargetStreamer());
 
   TS->EmitDirectiveHSACodeObjectVersion(2, 1);
 
+  const MCSubtargetInfo *STI = TM.getMCSubtargetInfo();
   AMDGPU::IsaVersion ISA = AMDGPU::getIsaVersion(STI->getFeatureBits());
   TS->EmitDirectiveHSACodeObjectISA(ISA.Major, ISA.Minor, ISA.Stepping,
                                     "AMD", "AMDGPU");
 
   // Emit runtime metadata.
   TS->EmitRuntimeMetadata(M);
 }