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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

I'm not sure this is a correct fix for the linked issue. Why exactly are we ending up with a negative estimated loop trip count?

I'm also wondering if we should reject negative estimated loop trip counts in the verifier.

CC @jdenny-ornl 

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