[MooreToCore] OOB array slices of unpacked arrays lowered like packed arrays

If I understand IEEE 1800-2017 §7.4.6 Indexing and slicing of arrays right, an unpacked array slice should be 0/X in its entirety even if only a part of the slice is OOB. Currently, only the OOB part is 0/X because they are lowered just like packed arrays.

Start at the MooreToCore lowering and trace how unpacked array slices are handled when an index is out of bounds. Compare the behavior with IEEE 1800-2017 §7.4.6 and verify that a slice containing any OOB portion produces 0/X in its entirety.

Written by the indexing model from the issue text.