[llvm] [DAG] isKnownNeverNaN - add DemandedElts element mask to isKnownNeverNaN calls (PR #135952)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

Ideally we'd move all this to be like the IR computeKnownFPClass too 

https://github.com/llvm/llvm-project/pull/135952