Put an llvm_unreachable at the end of getSplatIndex as its loop should never find all undef elements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187775 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -1205,7 +1205,7 @@ public:
       if (Mask[i] >= 0)
         return Mask[i];
     }
-    return -1;
+    llvm_unreachable("Splat with all undef indices?");
   }
   static bool isSplatMask(const int *Mask, EVT VT);