[llvm] [SelectionDAG] Do not build illegal nodes with users (PR #108573)

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ErikHogeman wrote:

> @ErikHogeman That IR is building OK for me - what are your command args? Can you repro it in godbolt?

Hi, apologies if I wasn't clear in the comments, I have not been able to reproduce it on an upstream target. This IR was the closest I got since it builds the invalid node, but as I wrote it happens too early (needs to happen after LegalizeDAG), plus another optimization needs to insert the illegal node back to the DAG combiner worklist before it gets removed later. I tried to explain what happens in my first comment, so I have been trying to write something that triggers those same steps in an upstream target.

https://github.com/llvm/llvm-project/pull/108573