isl_tab_detect_redundant: extract out select_marked
isl_tab_detect_redundant: extract out select_marked

This reduces the size of isl_tab_detect_redundant and makes it
easier to change the policy of which variable is considered first.

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

PollyLabs/isl,BenzoSM/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,Meinersbur/isl,nicolasvasilache/isl,inducer/isl-mirror,BenzoSM/isl,UBERTC/isl,UBERTC/isl,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,UBERTC/isl,tobig/isl,nicolasvasilache/isl,BenzoSM/isl,nicolasvasilache/isl,tobig/isl,BenzoSM/isl,nicolasvasilache/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,tobig/isl,BenzoSM/isl,tobig/isl,inducer/isl-mirror,UBERTC/isl