isl_mat_variable_compression: extract out empty_compression
isl_mat_variable_compression: extract out empty_compression

This reduces the size of isl_mat_variable_compression and
makes it easier to see the differences in the next commit.

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

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