[llvm] [InstCombine] Canonicalize (a + 1 == 0) ? -1 : a + 1 -> uadd.sat(a, 1) (PR #144566)

[via llvm-commits]

AZero13 wrote:

> I don't think this is the right place to fix this. We need to rewrite
> 
> ```
>   %add = add i32 %num, 1
>   %cmp = icmp eq i32 %add, 0
> ```
> 
> to
> 
> ```
>   %cmp = icmp eq i32 %num, -1
> ```
> 
> Then the rest just works.
unsigned square2eer(unsigned a) {
    return a + 7 == 7;
}

folds fine, but when we have multiple uses, it is not simplifying because it is reusing the a + 7 

https://github.com/llvm/llvm-project/pull/144566