[llvm] [NFC] NFC patch to lockdown instructions of vspltisw for addition of vector of 1s (PR #160476)

[via llvm-commits]

Himadhith wrote:

> Is there an existing PR that will optimize this code gen?

[[PowerPC] Replace vspltisw+vadduwm instructions with xxleqv+vsubuwm for adding the vector {1, 1, 1, 1}](https://github.com/llvm/llvm-project/pull/160882#top)
I was still working on this optimization when I posted the NFC patch.

https://github.com/llvm/llvm-project/pull/160476