add option to selectively disable variable solving for only ground expressions

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/ast/simplifiers/solve_eqs.h

@@ -42,6 +42,7 @@ namespace euf {
             bool m_context_solve = true;
             unsigned m_max_occs = UINT_MAX;
             bool m_enabled = true;
+            bool m_enable_non_ground = true;
         };
 
         stats                         m_stats;