dict_col_get_clust_pos(): Add parameter clust_index.  Replace the
look-up with a linear search of all columns in the clustered index.

row_upd_index_replace_new_col_vals(): Compute clust_index outside
the loops.  Compute clust_pos outside the inner loop.

row_upd_changes_ord_field_binary(), row_upd_changes_first_fields_binary():
Compute clust_index outside the loops.  Declare the auxiliary variables
inside the loop scope.