[llvm] [AArch64] Improve lowering for scalable masked deinterleaving loads (PR #154338)

[Cullen Rhodes via llvm-commits]

https://github.com/c-rhodes approved this pull request.

thanks for updates, sorry for the noise pointing out all the untested codepaths in the first round of review, it's just there's a ton of constraints in the pattern that's pretty overwhelming so I went through and added unreachables to figure out what was what, and noted ones that didn't fire.

I've got some more suggestions to improve the tests, mostly minor, but the actual code changes LGTM, cheers!

https://github.com/llvm/llvm-project/pull/154338