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

[via llvm-commits]

modiking wrote:

@nikic Appreciate the thorough review on the best way to get this done. Right now, with locking the full unroll under Pragma the repro unrolls 2048 times which is fine but fairly slow to complete (155s on my SkyLake).

Thoughts on a way to cap this in some way or is this alright?

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