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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 1. Feel free to extend [Document GlobalISel's combiner guidelines #92309](https://github.com/llvm/llvm-project/issues/92309)
> 2. We combine select(cmp) into min/max.
> 3. We combine select into binary operators.

These are all target dependent optimization choices. Ideally we would have a library of combines in both directions, and the target is free to enable whichever set it wants. We could have an IR-esque canonicalization and de-canonicalization sequence of combiner passes

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