[llvm] [LV] Add support for uniform parameters on vectorized function variants (PR #68879)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

I've removed the uniform argument case and the tests from this PR, so it should just be NFC refactoring in preparation now. I'll precommit the tests then raise a new PR for the uniform case. Ok?

https://github.com/llvm/llvm-project/pull/68879