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

[via llvm-commits]

modiking wrote:

> Though I'm surprised that unrolling to 2048 times takes 155s -- is this with a debug build of LLVM by chance?

I checked again and I think you're right on that, with a release build it's much faster:
```
~/llvm-project/llvm/test/Transforms/LoopUnroll# time ~/llvm-project/build-rel/bin/llvm-lit pr77842.ll 
-- Testing: 1 tests, 1 workers --
PASS: LLVM :: Transforms/LoopUnroll/pr77842.ll (1 of 1)

Testing Time: 6.25s

Total Discovered Tests: 1
  Passed: 1 (100.00%)

real    0m6.486s
user    0m6.430s
sys     0m0.083s
```

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