[VPlan] Improve code using m_One (NFC) (llvm#161686)

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -3346,12 +3346,7 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan) {
         VectorStep = Builder.createWidenCast(CastOp, VectorStep, IVTy);
       }
 
-      [[maybe_unused]] auto *ConstStep =
-          ScalarStep->isLiveIn()
-              ? dyn_cast<ConstantInt>(ScalarStep->getLiveInIRValue())
-              : nullptr;
-      assert(!ConstStep || ConstStep->getValue() != 1);
-      (void)ConstStep;
+      assert(!match(ScalarStep, m_One()) && "Expected non-unit scalar-step");
       if (TypeInfo.inferScalarType(ScalarStep) != IVTy) {
         ScalarStep =
             Builder.createWidenCast(Instruction::Trunc, ScalarStep, IVTy);