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

[David Blaikie via llvm-commits]

dwblaikie wrote:

It might be that this test times out if the loop isn't fully unrolled. (removing the unroll pragma causes the program to timeout - FWIW, this is is something BPF related - there's some comment about the pragma being required for some verification - I don't really understand BPF enough to explain it)

If I remove this part of your change:
```
if (OnlyFullUnroll && (UP.Count < TripCount || UP.Count < MaxTripCount)) {
```
And switch it back to the old code, the program finishes in roughly the expected time (~3-4 minutes, rather than hitting our 10min test timeout).

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