[AArch64][PAC] Fix clobbering registers by BLRA and AUTH_TCRETURN (#155373) After `isX16X17Safer` predicate was introduced, it became possible for `emitPtrauthDiscriminator` function to return its `AddrDisc` argument when it is neither X16 nor X17 (which are declared as implicit-def'ed by `BLRA` and `AUTH_TCRETURN[_BTI]` pseudo instructions). This resulted in the above pseudos being able to accidentally clobber unexpected register. As a quick fix for miscompilation possibility, this patch virtually restores the old behavior for the affected pseudo instructions. It should be possible to improve the efficiency via subsequent patches by accounting for `killed` flags and register masks.
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