isl_basic_map_zip: always call isl_basic_map_cow
isl_basic_map_zip: always call isl_basic_map_cow

We were relying on isl_basic_map_swap_vars calling isl_basic_map_cow for us,
but if no actual swapping needs to be performed (because the number
of variables to swap is zero), then isl_basic_map_swap_vars will not call
isl_basic_map_cow.

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

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