[llvm] [AArch64] Tighten conditions for expanding GET_ACTIVE_LANE_MASK (PR #208962)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

It looks like some cost model work is required but that's not the fault of this PR.

https://github.com/llvm/llvm-project/pull/208962