[llvm] [DAG] SelectionDAG::isKnownToBeAPowerOfTwo - add ISD::TRUNCATE handling and tests (PR #184365)

[Simon Pilgrim via llvm-commits]

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

Missing vector coverage and tests for the OrZero == false path

https://github.com/llvm/llvm-project/pull/184365