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

[Arda Serdar Pektezol via llvm-commits]

pektezol wrote:

> (This code should also treat it as unsigned, not int...)

`getOptionalIntLoopAttribute` does return an int if it is specified in the metadata though.

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