isl_tab_basic_map_partial_lexopt: don't normalize intermediate div expressions

isl_tab_basic_map_partial_lexopt: don't normalize intermediate div expressions

The function get_row_parameter_div would call normalize_div before finishing
the construction of the div expression.  In particular, it would call
normalize_div before negating the coefficients.  Since normalize_div may round
down the constant term, the result of first normalizing and then negating
may result in a different div than first negating and then normalizing.
Call normalize_div only after negating the coefficients.

Reported-by: Alexandre Isoard <alexandre.isoard@ens-lyon.fr>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>