[llvm] [LLVM][LangRef] Remove "n > 0" restriction from get.active.lanes.mask. (PR #152140)

[Sam Parker via llvm-commits]

sparker-arm wrote:

> Does this sounds believable?

We certainly rely on separate intrinsic to set the trip count but, IIRC, SCEV is used to calculate whether the two intrinsics agree on the expected behaviour. If this guarantee isn't meant, then we rely on  SelectionDAG lowering, like everyone else, which happens [here](https://github.com/llvm/llvm-project/blob/5499a70c39bfea10a0139ed6e98a267b9473448d/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp#L8057).

https://github.com/llvm/llvm-project/pull/152140