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

[Philip Ginsbach-Chen via llvm-commits]

ginsbach wrote:

> This seems reasonable to me 👍 I'm just going to tag @davemgreen too since he worked on the ZIP patch.

Thank you very much!

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.

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

> The following files introduce new uses of undef:
> 
> * llvm/test/CodeGen/AArch64/arm64-trn.ll

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

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