isl_tab_pip.c: add_div: use more reliable way to test whether div is nonneg
isl_tab_pip.c: add_div: use more reliable way to test whether div is nonneg

Before, we used the fact that if all parameters are non-negative,
then the big parameter is not used.
However, it is perfectly valid to have negative parameters and
still not use any big parameter.

UBERTC/isl,crossbuild/isl,simbuerg/isl,PollyLabs/isl,tobig/isl,evaautomation/isl,UBERTC/isl,evaautomation/isl,abduld/isl,nicolasvasilache/isl,serge-sans-paille/isl,epowers/isl,tobig/isl,VanirLLVM/toolchain_isl,KangDroidSMProject/ISL,cfx-next/toolchain_isl-upstream,Meinersbur/isl,epowers/isl,tobig/isl,abduld/isl,BenzoSM/isl,simbuerg/isl,KangDroidSMProject/ISL,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,VanirLLVM/toolchain_isl,Meinersbur/isl,KangDroidSMProject/ISL,cfx-next/toolchain_isl-upstream,crossbuild/isl,inducer/isl-mirror,serge-sans-paille/isl,evaautomation/isl,BobSaget-Mod/libisl,pierrotdelalune/isl,simbuerg/isl,pierrotdelalune/isl,Distrotech/isl,jleben/isl,jleben/isl,epowers/isl,abduld/isl,cfx-next/toolchain_isl-upstream,epowers/isl,SaberMod/isl-current,jleben/isl,KangDroidSMProject/ISL,jleben/isl,KangDroidSMProject/ISL,BobSaget-Mod/libisl,BenzoSM/isl,nicolasvasilache/isl,UBERTC/isl,nicolasvasilache/isl,pierrotdelalune/isl,cfx-next/toolchain_isl-upstream,jleben/isl,BenzoSM/isl,BobSaget-Mod/libisl,Distrotech/isl,serge-sans-paille/isl,Distrotech/isl,serge-sans-paille/isl,inducer/isl-mirror,Distrotech/isl,VanirLLVM/toolchain_isl,VanirLLVM/toolchain_isl,BobSaget-Mod/libisl,crossbuild/isl,inducer/isl-mirror,inducer/isl-mirror,pierrotdelalune/isl,epowers/isl,abduld/isl,Distrotech/isl,PollyLabs/isl,PollyLabs/isl,tobig/isl,BenzoSM/isl,SaberMod/isl-current,BenzoSM/isl,SaberMod/isl-current,Meinersbur/isl,BobSaget-Mod/libisl,VanirLLVM/toolchain_isl,pierrotdelalune/isl,nicolasvasilache/isl,serge-sans-paille/isl,SaberMod/isl-current,evaautomation/isl,simbuerg/isl,PollyLabs/isl,simbuerg/isl,cfx-next/toolchain_isl-upstream,crossbuild/isl,UBERTC/isl,nicolasvasilache/isl