[llvm] [DAG] SimplifyDemandedBits - ensure we demand the high bits for shl nsw/nuw ops (PR #70041)

[Nikita Popov via llvm-commits]

nikic wrote:

It looks like SDAG also follows the same approach in other cases, e.g. https://github.com/llvm/llvm-project/blob/c0914b222b1cdf3ab6bfabdec14ed1d0ef865765/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp#L2732-L2738. I don't really see why we treat `shl` as a special snowflake.

https://github.com/llvm/llvm-project/pull/70041