WIP: Generalize control flow analysis for infinite iteration

[samarthkishor]

Builds on the work in #186

• fixes the issue where sum(a for a in NeverTerminate(::Int))
  did not report an error
• refactors iteration termination check into a separate function
• adds another function to find iteration info for a BasicBlock

[aviatesk]

Thanks ! Don't you mind if I push a commit to this branch ? (e.g. resolve conflict or such)

[samarthkishor]

Thanks ! Don't you mind if I push a commit to this branch ? (e.g. resolve conflict or such)

Sure, go ahead!

[aviatesk]

Sorry I couldn't manage to update this branch as I expected, maybe because this branch has the same base name as the target (i.e. iteration).
I opened yet another PR to take your changes in: #200

I will review the changes there, and if you want to make changes to the branch, please make another PR on top of it.