Issue 181645
Summary [DAG] isKnownToBeAPowerOfTwo - add DemandedElts + OrZero handling to ISD::SELECT/VSELECT cases
Labels good first issue, llvm:SelectionDAG
Assignees
Reporter RKSimon
    The select nodes can safely filter elements using DemandedElts and can correctly identify zero cases when OrZero is true:

https://github.com/RKSimon/llvm-project/blob/20aff2085c358d3e112b245494161ddb9ffe0410/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L4748-L4753

The actual change to the inner isKnownToBeAPowerOfTwo calls is trivial (add DemandedElts + OrZero passthrough args), but this will require test coverage as well - these might be possible by extending existing DAG tests, else maybe add unit tests like we did in https://github.com/llvm/llvm-project/pull/181485
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