[Vectorize] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp:2417:13: error:
  unused variable 'ConstStep' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -2419,6 +2419,7 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan,
               ? dyn_cast<ConstantInt>(ScalarStep->getLiveInIRValue())
               : nullptr;
       assert(!ConstStep || ConstStep->getValue() != 1);
+      (void)ConstStep;
       if (TypeInfo.inferScalarType(ScalarStep) != IVTy) {
         ScalarStep =
             Builder.createWidenCast(Instruction::Trunc, ScalarStep, IVTy);