Fixes internal compiler error while compiling with VC2015 Update1 x64.

Eigen/src/Core/products/GeneralMatrixMatrix.h

@@ -140,8 +140,10 @@ static void run(Index rows, Index cols, Index depth,
       // Release all the sub blocks B'_j of B' for the current thread,
       // i.e., we simply decrement the number of users by 1
       for(Index j=0; j<threads; ++j)
+      {
         #pragma omp atomic
-        --(info[j].users);
+        info[j].users -= 1;
+      }
     }
   }
   else