isl_map_simplify.c: remove_duplicate_divs: preserve known divs

isl_map_simplify.c: remove_duplicate_divs: preserve known divs

As explained in the commit message of 22c210c (isl_basic_map_gauss:
try not to remove any div definitions, Tue Sep 1 22:59:21 2009 +0200),
we should preserve known divs during isl_basic_map_simplify.
That commit only fixed the possible loss of known divs in
isl_basic_map_gauss.  Here we fix the same problem in remove_duplicate_divs.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>