[llvm] Do not use R12 for indirect tail calls with PACBTI (PR #82661)

[via llvm-commits]

ostannard wrote:

> They are all in computeAvailableFunctionFeatures() (when built) and I struggled to see this function being executed.

I see copies of this code being built into `computeAvailableFunctionFeatures` (in `AArch64GenGlobalISel.inc`), and in `CheckPatternPredicate` (in `AArch64GenDAGISel.inc`), have you checked that latter? Global isel isn't enabled by default for 32-bit ARM, so I'd expect DAGISel to be the important one here.

https://github.com/llvm/llvm-project/pull/82661