Use pop_back_val() instead of both back() and pop_back().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188723 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineWorklist.h

@@ -74,8 +74,7 @@ public:
   }
 
   Instruction *RemoveOne() {
-    Instruction *I = Worklist.back();
-    Worklist.pop_back();
+    Instruction *I = Worklist.pop_back_val();
     WorklistMap.erase(I);
     return I;
   }