[llvm] [AArch64][PAC] Rework the expansion of AUT/AUTPAC pseudos (PR #169699)

[Anatoly Trosinenko via llvm-commits]

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@@ -2340,46 +2332,80 @@ void AArch64AsmPrinter::emitPtrauthAuthResign(
     break;
   }
 
-  // Compute aut discriminator
-  Register AUTDiscReg =
-      emitPtrauthDiscriminator(AuthSchema.IntDisc, AuthSchema.AddrDisc, Scratch,
-                               AuthSchema.AddrDiscIsKilled);
+  // Checked-but-not-trapping mode ("poison") only applies to resigning,
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atrosinenko wrote:

As far as I understand, there are a lot of such test cases in [`ptrauth-intrinsic-auth-resign-with-blend.ll`](https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/AArch64/ptrauth-intrinsic-auth-resign-with-blend.ll) (the check lines have `CHECKED` prefix) - the trick is that `Poison` is technically the **default** mode, but it is usually overridden to `Trap` by specifying `"ptrauth-auth-traps"` function attribute.

https://github.com/llvm/llvm-project/pull/169699