[llvm] [SelectionDAG] Add an ISD node for vector.extract.last.active (PR #118810)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

Back to working on this. Latest patch is just a rebase, but I'll start experimenting with the different approaches I mentioned last year to see if those feel less hacky. I think we'll still need to look into `getBitWidthForCttzElements`, as making VTs based on the returned value often causes problems (nodes that can't be legalized for targets with scalable vectors).

https://github.com/llvm/llvm-project/pull/118810