[llvm] [AArch64] match TRN starting from undef elements (PR #167955)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:


> I have consolidated the changes into a single commit according to [the guidelines](https://llvm.org/docs/GitHub.html#landing-your-change), with message and title matching the PR. There were no other changes in the force-push.

No worries, but I think those docs are a little outdated/misleading; there's no need for you to manually squash your commits before a PR is merged. 

> I expect that this is fine to ignore, as it matches the approach in the remainder of this file?

`undef` is being phased out. It would be better if your new tests used `poison`, since this will need to be changed eventually. I don't have a strong preference, though.

> I don't have write access, so won't be able to merge myself.

I'll merge this in a day or so (if there's no further comments). 


https://github.com/llvm/llvm-project/pull/167955