[llvm] [AArch64] Fix return address auth in swiftasync epilogues (PR #189484)

[Jon Roelofs via llvm-commits]

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jroelofs wrote:

Good point!

A couple things help out here in aapcs / darwinpcs:
* X16/X17 are linker scratch registers, and will be dead at this point in epilogues.
* X15 is caller-saved, so it's neither an argument nor a return reg, and must also be dead.

I think the main thing to worry about is the register used for indirect tail calls.

https://github.com/llvm/llvm-project/pull/189484