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

[via llvm-commits]

modiking wrote:

@nikic Change passes unit tests, also checked that both are needed as it fails a unit test if we only switch to `UP.Count < TripCount`

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