[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.

This shouldn't be in my proof. I apologize: it's actually malformed IR.

nuw is impossible for x ugt -C ? x + C : -1;

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