Add semantic loop unrolling analysis

[sim642]

Frustrated with the syntactic loop unrolling (#563), this is a quick shot at a semantic one that I've wanted for a long time: just use path sensitivity to count and keep loop iterations separate.

The benefits are obvious:

1. This is a lot simpler than Loop Unrolling for the first exp.unrolling-factor Iterations #563.
2. It automatically provides a clean fix to Witness invariants for unrolled loops are incorrect #1225, etc.
3. We avoid creating large ASTs, CFGs and constraint systems, especially if the loop has a finite bound and all subsequent iterations are dead.
4. It allows the unrolling factor to depend on any semantic information that we want because it can be determined during the analysis.

TODO

• Figure out interprocedural behavior (contexts, enter, combine).
• Integrate with autotuning again.
• Check sv-benchmarks results.