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

[David Sherwood via llvm-commits]

david-arm wrote:

> > 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?
> 
> Thanks for the comment, I will be adding test cases soon (probably next few days)

Yep, thanks it would be good to add test cases! There are some interesting cases, such as vectorising an outer loop but it requires a pragma.

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