polyhedron_sample.c: avoid use of isl_vec internals
polyhedron_sample.c: avoid use of isl_vec internals

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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