Silence sign-compare warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154297 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index f1b9f43..a882295 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7824,7 +7824,7 @@
         Idx = OtherSV->getMaskElt(Idx);
 
       // The combined shuffle must map each index to itself.
-      if (Idx != i && Idx != -1)
+      if ((unsigned)Idx != i && Idx != -1)
         return SDValue();
     }