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

[via llvm-commits]

ErikHogeman wrote:

@bogner , I believe you are listed as the code owner of the SelectionDAG part of LLVM. If/when you have time, could you help me add the relevant people to review this change?

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