[InstCombine] Erase old instruction when replacing extractelements

As we are not returning the result of replaceInstUsesWith(),
so we need to clean up ourselves.

NFC apart from worklist order.

lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -597,6 +597,7 @@ static void replaceExtractElements(InsertElementInst *InsElt,
     auto *NewExt = ExtractElementInst::Create(WideVec, OldExt->getOperand(1));
     NewExt->insertAfter(OldExt);
     IC.replaceInstUsesWith(*OldExt, NewExt);
+    IC.eraseInstFromFunction(*OldExt);
   }
 }