[llvm] [IVDescriptors] Implement MonotonicDescriptor (PR #214490)

[Andrei Elovikov via llvm-commits]

https://github.com/eas commented:

I think it's still very hard to read/reason about. We have a bunch of legality bailouts spread across two (if not three) functions, separated by the actual implementation of the bailout, as in 

```
// Comment what's the bailout condition, concise
... Several 
... lines
... implementing/verifying
...
...
....
...

// Another bailout multiple lines away in the same pattern
```

That means there is no single place to see all the necessary/required conditions in one place, requiring the reader to keep two screens of code in mind at the same time to reason about this. I don't think it's viable.

https://github.com/llvm/llvm-project/pull/214490