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

[Jon Roelofs via llvm-commits]

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jroelofs wrote:

Sorry, I misspoke and that made this confusing. I should have said:

> PACSym needs to be materialized into `x15`

There are a few cases to consider:

* `MFnI->branchProtectionPAuthLR() && Subtarget->hasPAuthLR()` - here we should have the pure pauth-lr instructions only, i.e. `autia171615`
* `MFnI->branchProtectionPAuthLR() && !Subtarget->hasPAuthLR()` - here we should have the `hint` instruction `pacm` that tells `AUTIA1716` to behave as if it were `AUTIA171615` on hardware that supports pauth-lr. On hardware that does not support pauth-lr, that hint instruction is a no-op. Therefore in this case, we need the PACSym in x15, and it's simply ignored when it's not needed.
* `!MFnI->branchProtectionPAuthLR()` - here we should have the `AUTIA1716`, but it is unnecessary to put PACSym in `x15`.

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