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

[via llvm-commits]

modiking wrote:

> I still think you should use `-pragma-unroll-threshold` in the test to limit the size. It's not really useful to have a 12-thousand line test vs one that tests the same thing on smaller limits.
> 
> A possible alternative would be to use the `loop-unroll-full` pass instead of the `loop-unroll` pass. I think that one should just stop unrolling entirely after your change.

Makes sense. Good suggestion with `loop-unroll-full`, confirmed it fails without this PR and works correctly with 

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