[llvm] Reserve R9 on armv6 iOS 2.x (PR #150835)

[Jon Roelofs via llvm-commits]

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

I haven't done the "archaeology" to check this, but it seems fine: IIUC the worst case fallout is that this hurts performance on a no-longer-supported platform, and best case is that it fixes this bug... so the tradeoff seems fine in light of that.

https://github.com/llvm/llvm-project/pull/150835