[llvm] [DAG] Always allow folding XOR patterns to ABS pre-legalization (PR #94601)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> It has always felt funny to me for DAG to create nodes that the target doesn't have support for. I guess it makes sense for abs.
> 
> > @davemgreen I've had to pull out a few ARM-specific changes from #94504 to prevent infinite loops re-expanding vXi64 ABS.
> 
> > We've lost the neon vabdl.u32 pattern, do you want me to tweak the neon tablegen pattern in this patch? It will be removed entirely in #94504 which should be ready to go immediately after this patch.
> 
> Do you mean that this and #94504 cancel each other out and the regression goes away? If so this sounds fine, yeah.

Thanks - I've simplified #94504 now so would prefer to land that first if you are happy to accept it.

https://github.com/llvm/llvm-project/pull/94601