[llvm] [SelectionDAG] Add an ISD node for for get.active.lane.mask (PR #139084)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

Given many of the changes are required to maintain existing code quality after loosing the lowering to the `whilelo` intrinsic I think it's worth seeing if you can also change `performIntrinsicCombine` to always lower the intrinsic itself to `GET_ACTIVE_LANE_MASK`. That way all the code paths will be unified and we'll not need extract isel patterns because the `GET_ACTIVE_LANE_MASK` can replace the existing intrinsic ones?

https://github.com/llvm/llvm-project/pull/139084