[llvm] [LoopVectorize][NFC] Centralize the setting of CostKind (PR #121937)

[David Sherwood via llvm-commits]

david-arm wrote:

> > Thanks for making the changes - I left one more comment about a couple more cases I think are missing and then it looks good to go!
> 
> I don't see any new comments in the gitub ui?

Doh! Sorry, the comment was still marked as pending and I forgot to submit it. Done it now.

https://github.com/llvm/llvm-project/pull/121937