[llvm] [LLVM][SVE] Improve legalisation of fixed length get.active.lane.mask (PR #90213)

[David Sherwood via llvm-commits]

https://github.com/david-arm commented:

A work of art @paulwalker-arm! I just had one minor suggestion, but if you don't agree with my comment then I'll just accept it.

https://github.com/llvm/llvm-project/pull/90213