isl_id_alloc: allow isl_ids with the same name but different user value
isl_id_alloc: allow isl_ids with the same name but different user value

Although isl_ids were documented as being identified by the pair
of name and pointer value, this was not implemented as such.
Instead, only the name was taken into account.

This commit fixes isl_id_alloc to also take into account the user value.
We also avoid dereferencing possible NULL names.

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

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