[BasicAA] Unify struct/other offset (NFC)
The distinction between StructOffset and OtherOffset has been
originally introduced by 82069c44ca39df9d506e16bfb0ca2481866dd0bb,
which applied different reasoning to both offset kinds. However,
this distinction was not actually correct, and has been fixed by
c84e77aeaefccb8d0c4c508b8017dcad80607f53. Since then, we only ever
consider the sum StructOffset + OtherOffset, so we may as well
store it in that form directly.