[AMDGPU] Fix a warning

This patch fixes:

  llvm/lib/Target/AMDGPU/AMDGPUIGroupLP.cpp:255:18: error: private
  field 'DAG' is not used [-Werror,-Wunused-private-field]

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPUIGroupLP.cpp

@@ -252,7 +252,7 @@ using SUsToCandSGsVec = SmallVector<SUToCandSGsPair, 4>;
 // only be used for small sized problems or medium sized problems where an exact
 // solution is highly desired.
 class PipelineSolver {
-  ScheduleDAGMI *DAG;
+  [[maybe_unused]] ScheduleDAGMI *DAG;
 
   // Instructions that can be assigned to multiple SchedGroups
   DenseMap<int, SUnitsToCandidateSGsMap> SyncedInstrs;