isl_flow.c: compute_val_based_dependences: make error checking more explicit
isl_flow.c: compute_val_based_dependences: make error checking more explicit

In particular, check whether the result of intermediate_sources
is smaller than 0 rather than simply different from zero.
A negative result is a stronger indication of an error condition
than a non-zero result, which could be confused with a positive result.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,PollyLabs/isl,nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl,Meinersbur/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,nicolasvasilache/isl