[llvm] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Christian Kissig via llvm-commits]

christiankissig wrote:

I've addressed final comments, rebased the changes and removed merge commits, reran all tests, but seem to lack permissions

```
> git push -f origin main
ERROR: Permission to llvm/llvm-project.git denied to christiankissig.
```

https://github.com/llvm/llvm-project/pull/67788