[llvm] [PPC][AIX] Save/restore r31 when using base pointer (PR #100182)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: 953d1f15991b5a49e232375b7dedb7bc60f72b6e

https://github.com/llvm/llvm-project/actions/runs/10372003179

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/100182