[llvm] [LoopUnroll] Introduce UnrollMaxIterations as a hard cap on how many iterations we try to unroll (PR #78648)

[via llvm-commits]

modiking wrote:

> > Change passes unit tests, also checked that both are needed as it fails a unit test if we only switch to `UP.Count < TripCount`
> 
> Yeah, currently we need both as they are mutually exclusive -- we only set MaxTripCount if TripCount is not available. Does make we wonder if it would make things cleaner if we just always computed MaxTripCount. But that's unrelated to this patch...

That would be cleaner. But yeah a different patch

https://github.com/llvm/llvm-project/pull/78648