[llvm] Spill/restore FP/BP around instructions in which they are clobbered (PR #81048)

[Reid Kleckner via llvm-commits]

https://github.com/rnk commented:

I'm finding myself short on time, so maybe I can hand the review off to @zmodem , who has done various backend changes to switch lowering and conditional tail calls and so on.

The current behavior is pretty bad, so even if there are some edge cases that this solution doesn't handle such as inaccurate unwind info, this will be an improvement, and we should try to move forward.

https://github.com/llvm/llvm-project/pull/81048