[llvm] [LoopUtils] prevent negative estimated trip count result (PR #195610)

[Arda Serdar Pektezol via llvm-commits]

pektezol wrote:

I have gone off from the IR reproducer, which could technically come from a fuzzer--and in this case I believe the PGO data in the linked issue is deliberately using the negative value. Rejecting negative counts also works.

https://github.com/llvm/llvm-project/pull/195610