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

[maerhart]

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.