isl_qpolynomial_drop_dims: properly handle nested spaces
isl_qpolynomial_drop_dims: properly handle nested spaces

For consistency, we remove the nesting even if n == 0.

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

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