[llvm] [ValueTracking] Fold max/min when incrementing/decrementing by 1 (PR #142466)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> Miscompilation reproducer: https://alive2.llvm.org/ce/z/nxGw_V
> 
> ```
> define i8 @src(i8 %x, i8 %w) {
>   %cmp = icmp ugt i8 %x, %w
>   %add = add nsw nuw i8 %w, 1
>   %r = select i1 %cmp, i8 %x, i8 %add
>   ret i8 %r
> }
> 
> define i8 @tgt(i8 %x, i8 %w) {
>   %add = add nsw nuw i8 %w, 1
>   %r = call i8 @llvm.umax(i8 %x, i8 %add)
>   ret i8 %r
> }
> ```
> 
> We need to drop nsw/nuw flags in this case.

Thanks! I've fixed this case by ensuring that we now drop the no-wrap flag which doesn't correspond to the sign of the min/max. I've added tests to confirm as well. 

https://github.com/llvm/llvm-project/pull/142466