[llvm] [WIP][DAG] Introduce generic shl_add node [NFC] (PR #88791)

[Philip Reames via llvm-commits]

preames wrote:

> > This patch is not a NFC.
> > Example: https://godbolt.org/z/PMTqjf45M
> 
> Looks like maybe I missed a pattern for the .uw variant, will go track this down.

I've confirmed this was a missing pattern, and have a local patch with a fix separated based on the RISCVISD::SHL_ADD change.  I am not going to post it until the RISCVISD::SHL_ADD patch lands to avoid confusing us all. 

https://github.com/llvm/llvm-project/pull/88791