[llvm] [ARM] r11 is reserved when using -mframe-chain=aapcs (PR #86951)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

This isn't really consistent with how we handle frame-pointer options on other targets.

On the target-independent side: there are three possible settings for the frame pointer attribute/flag: "all", "non-leaf", and "none".  In "all" and "non-leaf" mode, we want to preserve the frame pointer register; in "none" mode, we don't.  If we do want a "don't create a frame pointer, but reserve the register" mode, I think we should change the way the target-independent settings work.

That isn't to say the change to ARMBaseRegisterInfo.cpp is wrong, but I'm not sure the implementation of isFPReserved() works the way we want.

https://github.com/llvm/llvm-project/pull/86951