[llvm] [LoopVectorize] Avoid considering PHI without any usage. (PR #92399)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

If there are PHI nodes that have no uses, I would expect those nodes to be removed by DCE. The tests that you've modified probably show up with changes because they're being passed straight into the loop-vectorize pass. In a normal use-case I would have expected those PHI nodes to have been deleted. So I'm not sure if this change is useful?

https://github.com/llvm/llvm-project/pull/92399