[llvm] [LV] Fix handleFirstArgMinOrMax to use 0 as identity element for UMax/SMax (PR #192054)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> PoisonValue, which can result in undefined behavior if the strict predicate is never met

Could you provide more details about what kind of UB this would expose? The poison should never leak, as the final select should propagate the original start value

https://github.com/llvm/llvm-project/pull/192054