[AArch64][PAC] Rework the expansion of AUT/AUTPAC pseudos
Refactor `AArch64AsmPrinter::emitPtrauthAuthResign` to improve
readability and fix the conditions of `emitPtrauthDiscriminator` being
allowed to clobber AddrDisc:
* do not clobber `AUTAddrDisc` when computing `AUTDiscReg` on resigning
if `AUTAddrDisc == PACAddrDisc`, as it would prevent passing raw,
64-bit value as the new discriminator
* mark the `$Scratch` operand of `AUTxMxN` as early-clobber (fixes
assertions when emitting code at `-O0`)
* move the code computing `ShouldCheck` and `ShouldTrap` conditions to a
separate function
* define helper `struct PtrAuthSchema` to pass arguments to
`emitPtrauthAuthResign` in a better structured way
