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

[Nikita Popov via llvm-commits]

nikic wrote:

We really don't care about verbosity -- only about easy to regenerate tests and clear diffs. Using CHECK-NOT means it's not possible to regenerate the test, which means you lose the primary benefit of using UTC in the first place. If you have to manually fix up the results, you may as well stay with hand-written check lines.

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