[llvm] [AArch64] Fix active.lane.mask(0, cttz.elts(x)) -> 'brkb' transform (PR #180177)

[Paul Walker via llvm-commits]

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

A couple of minor issues but otherwise looks good to me.

https://github.com/llvm/llvm-project/pull/180177