[Vectorize] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp:1278:12: error:
  unused variable 'Op0' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1275,9 +1275,8 @@ void VPWidenEVLRecipe::execute(VPTransformState &State) {
   State.setDebugLocFrom(getDebugLoc());
   assert(State.UF == 1 && "Expected only UF == 1 when vectorizing with "
                           "explicit vector length.");
-  VPValue *Op0 = getOperand(0);
 
-  assert(State.get(Op0, 0)->getType()->isVectorTy() &&
+  assert(State.get(getOperand(0), 0)->getType()->isVectorTy() &&
          "VPWidenEVLRecipe should not be used for scalars");
 
   VPValue *EVL = getEVL();