isl_mat_scale_down: avoid trampling over aliased matrices
isl_mat_scale_down: avoid trampling over aliased matrices

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

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