[llvm] [VPlanValue] Introduce hasOneUser(). nfc (PR #170826)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

Closed since it's duplicated with https://github.com/llvm/llvm-project/pull/155397 and https://github.com/llvm/llvm-project/pull/157956.

https://github.com/llvm/llvm-project/pull/170826