[llvm] [DAG] SelectionDAG::canCreateUndefOrPoison - add ISD::SCMP/UCMP handling + tests (PR #154127)

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Temperz87 wrote:

> Can you add it in the same category as umax amd umin in ir and globalisel too since we are here already?

Is the same change that RKSimon suggested? 

https://github.com/llvm/llvm-project/pull/154127