MachineSink: Sort the successors before trying to find SuccToSinkTo.

Use stable_sort instead of sort. Follow-up to r161062.

rdar://11980766


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161075 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/MachineSink.cpp b/lib/CodeGen/MachineSink.cpp
index d02aa6f..bc383cb 100644
--- a/lib/CodeGen/MachineSink.cpp
+++ b/lib/CodeGen/MachineSink.cpp
@@ -538,7 +538,7 @@
       // we should sink to.
       // We give successors with smaller loop depth higher priority.
       SmallVector<MachineBasicBlock*, 4> Succs(MBB->succ_begin(), MBB->succ_end());
-      std::sort(Succs.begin(), Succs.end(), SuccessorSorter(LI));
+      std::stable_sort(Succs.begin(), Succs.end(), SuccessorSorter(LI));
       for (SmallVector<MachineBasicBlock*, 4>::iterator SI = Succs.begin(),
            E = Succs.end(); SI != E; ++SI) {
         MachineBasicBlock *SuccBlock = *SI;