[llvm] [LV] Emit better debug and opt-report messages when vectorization is disallowed in the LoopVectorizer (PR #158513)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen commented:

Is this patch NFC? If this is NFC, please mark NFC in the commit title; If not, could you add a test case to show the improvement?

https://github.com/llvm/llvm-project/pull/158513