[llvm] GlobalISel: Use G_UADDE when narrowing G_UMULH (PR #97194)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > In this case, I am strongly opposed. I don't want the combiner to create more legalization artefacts.
> 
> I don't follow. No legalization artifacts are mentioned here. It's an instruction, typically used for legalization, which should have a rich set of optimization combines on it. It is not a legalization artifact, nor are legalization artifacts something to be avoided

Plus this is a separate TODO combine, we're avoiding / delaying doing it by introducing UADDE from the beginning 


https://github.com/llvm/llvm-project/pull/97194