[llvm] [LV] Update LoopVectorizationPlanner::emitInvalidCostRemarks to handle reduction plans (PR #165913)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Thanks for the feedback. Updates:
- Test moved to AArch64 folder
- Test simplified to minimal example
- Variable names cleaned up for readability.
- Rebased onto more recent `main`

https://github.com/llvm/llvm-project/pull/165913