[llvm] Resolve FIXME: Swap adds <-> subs offset is 0 (PR #78870)

[David Green via llvm-commits]

davemgreen wrote:

Do you have tests cases for where this can come up? I'm not sure we should be generating instructions with negative offsets.
Could the add/sub do setting nzcv flags that are needed?

https://github.com/llvm/llvm-project/pull/78870