[Pipeliner] Use std::stable_sort when ordering NodeSets

There are cases when individual NodeSets can be equal with respect to
the ordering criteria. Since they are stored in an ordered container,
use stable_sort to preserve the relative order of equal NodeSets. 

This should remove non-determinism discovered by shuffling done in
llvm::sort with expensive checks enabled.

llvm-svn: 329915

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -931,7 +931,7 @@ void SwingSchedulerDAG::schedule() {
     }
   });
 
-  llvm::sort(NodeSets.begin(), NodeSets.end(), std::greater<NodeSet>());
+  std::stable_sort(NodeSets.begin(), NodeSets.end(), std::greater<NodeSet>());
 
   groupRemainingNodes(NodeSets);