isl_set_substitute: implement in terms of isl_set_preimage_multi_aff
isl_set_substitute: implement in terms of isl_set_preimage_multi_aff

isl_set_substitute is essentially a special case of
isl_set_preimage_multi_aff for plugging in a single purely affine
integer expression.  The implementation may be slightly
more efficient than the more general isl_set_preimage_multi_aff,
but it is not worth keeping around the extra code.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl