[Attributor][FIX] Avoid creating accidental poison callees

Back with f3ad8cf0 we introduced a bug that caused us to skip
callees when we replace uses. This is not sound since subsequent IR
cleanup will assume replacement has happend. As such we created poison
callees for a long while. The original intend of the check was to
prevent call graph invalidation, however, we now properly check if the
instructions (here the call) are inside the SCC or not.