[llvm] LoopVectorize/test: clean up reduction.ll; generate using UTC (NFC) (PR #68890)

[Nikita Popov via llvm-commits]

nikic wrote:

> Hardering CHECK-NOT lines to negative-match the vector.body label. This is the practice used in newer files such as iv-select-cmp.ll.

If you're using UTC, there should not be any CHECK-NOT lines.

https://github.com/llvm/llvm-project/pull/68890