[llvm] [DAG] Simplify isKnownToBeAPowerOfTwo for ZERO_EXTEND (PR #182226)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Once #184772 lands you should be able to remove the unit tests and regenerate known-pow2.ll

https://github.com/llvm/llvm-project/pull/182226