[llvm] [Statepoint] Treat result of atomicrmw xchg as a base pointer (PR #97280)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

(Note: I no longer work on this, so my knowledge is quite stale.  Approving only because this has been stuck in review for a very long time.)

https://github.com/llvm/llvm-project/pull/97280