[llvm] [PAC][InstCombine] Replace auth+sign with resign (PR #130807)

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls commented:

Naively, it would seem to me that it is a user error to not use the `resign` intrinsic and instead use `auth` and `sign` in sequence.
I can see that having this peephole optimization might reduce the occurrence of `auth` + `sign` operations in sequence, but it seems this is a "best effort" fix.
I wonder if you could share any insights into where these `auth` and `sign` sequences typically originate from. I wonder if other mitigations (e.g. generating warnings when seeing such sequences) wouldn't also be needed or sometimes be better?

https://github.com/llvm/llvm-project/pull/130807