isl_range.c: bound2poly: extract out signed_infty
isl_range.c: bound2poly: extract out signed_infty

We will be able to reuse this function in a subsequent commit.

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

KangDroidSMProject/ISL,Distrotech/isl,evaautomation/isl,nicolasvasilache/isl,UBERTC/isl,evaautomation/isl,Meinersbur/isl,inducer/isl-mirror,inducer/isl-mirror,UBERTC/isl,PollyLabs/isl,tobig/isl,Distrotech/isl,UBERTC/isl,PollyLabs/isl,simbuerg/isl,Distrotech/isl,Distrotech/isl,BenzoSM/isl,KangDroidSMProject/ISL,Distrotech/isl,inducer/isl-mirror,UBERTC/isl,simbuerg/isl,simbuerg/isl,evaautomation/isl,simbuerg/isl,Meinersbur/isl,tobig/isl,KangDroidSMProject/ISL,Meinersbur/isl,BenzoSM/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,BenzoSM/isl,Meinersbur/isl,PollyLabs/isl,simbuerg/isl,evaautomation/isl,tobig/isl,BenzoSM/isl,KangDroidSMProject/ISL,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl,inducer/isl-mirror,BenzoSM/isl,tobig/isl,PollyLabs/isl,nicolasvasilache/isl,PollyLabs/isl