gpu_group.c: compute_array_dim_size: improve error handling
gpu_group.c: compute_array_dim_size: improve error handling

In particular, start from an infinite bound to be able
to detect the difference between failure to find a bound and
failures that occur during the computation.

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

Meinersbur/ppcg,Meinersbur/ppcg