[AArch64][PAC] Refactor discriminator emission in AsmPrinter (NFC) (#160900)
Refactor the discriminator emission in `AArch64AsmPrinter`:
* factor out ad-hoc "X16 or X17 or not isX16X17Safer" checks into a
dedicated `isPtrauthRegSafe` function
* assert that `Disc` is uint16 once in `emitPtrauthDiscriminator`
instead of in all its callers
* update the comments and assertions for readability
* rename `MayUseAddrAsScratch` argument to `MayClobberAddrDisc`, as it
better reflects the intention