[llvm] [SelectionDAG] Add computeKnownBits support for ISD::VECREDUCE_ADD (PR #207631)

[Simon Pilgrim via llvm-commits]

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

RISCVISD::VECREDUCE_ADD_VL regressions needs to be addressed first

https://github.com/llvm/llvm-project/pull/207631