[!273] Fix a bug in 'pre' regarding the handling of scalar-valued matrices

Merge branch 'bugfix-for-amg-with-scalar-matrix' into 'master'

ref:core/dune-istl This bug is really new \-- it only came with the
introduction of support for scalar-valued matrices.

See merge request [!273]

  [!273]: gitlab.dune-project.org/core/dune-istl/merge_requests/273

dune/istl/paamg/amg.hh

@@ -587,7 +587,7 @@ namespace Dune
         {
           Hybrid::ifElse(IsNumber<Block>(),
             [&](auto id) {
-              x[row.index()] = id(diagonal) / b[row.index()];
+              x[row.index()] = b[row.index()] / id(diagonal);
             },
             [&](auto id) {
               id(diagonal).solve(x[row.index()], b[row.index()]);