isl_schedule.c: extract out merge_edge
isl_schedule.c: extract out merge_edge

Merging edges will become more complicated when we add support
for conditional validity constraints.  It is therefore best
to extract this functionality into a separate function.

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

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