[Cleanup] Avoid recomputation

dune/tectonic/mydirectionalconvexfunction.hh

@@ -45,8 +45,9 @@ template <class Nonlinearity> class MyDirectionalConvexFunction {
     Vector uxv = u;
     Arithmetic::addProduct(uxv, x, v);
     phi.directionalSubDiff(uxv, v, D);
-    D[0] += A * x - b;
-    D[1] += A * x - b;
+    auto const Axmb = A * x - b;
+    D[0] += Axmb;
+    D[1] += Axmb;
   }
 
   void domain(Dune::Solvers::Interval<double> &domain) const {