_slot_operator_update_probe: fix bug #508762

This fixes the check_reverse_dependencies function (inside the depgraph
_slot_operator_update_probe method) to account for irrelevant parent
atoms from parents that need to be rebuilt or have been involved in
unsolved slot conflicts.

X-Gentoo-Bug: 508762
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=508762
Reviewed-by: Brian Dolbec <dolsen@gentoo.org>