[BasicAA] Unify struct/other offset (NFC)

The distinction between StructOffset and OtherOffset has been
originally introduced by 82069c44,
which applied different reasoning to both offset kinds. However,
this distinction was not actually correct, and has been fixed by
c84e77ae. Since then, we only ever
consider the sum StructOffset + OtherOffset, so we may as well
store it in that form directly.