[llvm] [InstCombine] Canonicalize more saturated-add variants (PR #100008)

[Rose Silicon via llvm-commits]

RSilicon wrote:

> Your alive proof has
> 
> ```
> define dso_local i8 @src3(i8 %x, i8  %c) {
> entry:
>   %noZero = icmp ne i8 %c, 0
>   call void @llvm.assume(i1 %noZero)
>   %add = add nuw i8 %x, %c
>   %c.not = xor i8 %c, -1
>   %d = add i8 %c.not, 1
>   %cmp.not = icmp ugt i8 %x, %d
>   %cond = select i1 %cmp.not, i8 -1, i8 %add
>   ret i8 %cond
> }
> ```
> 
> The `nuw` flag there says that the add never overflows and thus never saturates. The transform fails without the nuw. There's no check for the nuw in your transform.

I meant to remove this. This exact situation is impossible to run into in real code. It is not covered by my code at all.

https://github.com/llvm/llvm-project/pull/100008