isl_basic_map_remove_divs_involving_dims: avoid adding constraints to copy

isl_basic_map_remove_divs_involving_dims: avoid adding constraints to copy

Before calling isl_basic_map_remove_dims,
isl_basic_map_remove_divs_involving_dims calls insert_bounds_on_div
to insert extra constraints that can be used during Fourier-Motzkin.
These constraints should not be added to the input of
the isl_basic_map_remove_divs_involving_dims call, but only to the output
(or rather the intermediate result that will be used to construct the output).
That is, add a call to isl_basic_map_cow to ensure any other copies
of the input are left alone.

We also clear the ISL_BASIC_MAP_NORMALIZED flag, but this should
not have any effect since the flag is also cleared by
isl_basic_map_drop.

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