Use llvm::stable_sort

While touching the code, simplify if feasible.

llvm-svn: 358996
diff --git a/llvm/lib/CodeGen/MachinePipeliner.cpp b/llvm/lib/CodeGen/MachinePipeliner.cpp
index fde6531..62760d2 100644
--- a/llvm/lib/CodeGen/MachinePipeliner.cpp
+++ b/llvm/lib/CodeGen/MachinePipeliner.cpp
@@ -428,7 +428,7 @@
     }
   });
 
-  std::stable_sort(NodeSets.begin(), NodeSets.end(), std::greater<NodeSet>());
+  llvm::stable_sort(NodeSets, std::greater<NodeSet>());
 
   groupRemainingNodes(NodeSets);