Clarify the semantics of `getAuthenticatedReg` and remove a redundant
`isAuthenticationOfReg` method, as combined auth+something instructions
(such as `retaa` on AArch64) should be handled carefully, especially
when searching for authentication oracles: usually, such instructions
cannot be authentication oracles and only some of them actually write an
authenticated pointer to a register (such as "ldra x0, [x1]!").
Use `std::optional<MCPhysReg>` returned type instead of plain MCPhysReg
and returning `getNoRegister()` as a "not applicable" indication.
Document a few existing methods, add information about preconditions.
e1328fd9ad