LoopUnswitching: Reimplement transformation

[phate]

This PR reimplements the loop unswitching transformation. It performs the following changes compared to the previous implementation:

1. Fixes a bug where the wrong origins for variables was picked up. In other words, the old transformation was broken.
2. Avoids the duplication of the exit subregion (the old transformation duplicates it)
3. Adds documentation to avoid confusion in the future.

Close #895

[haved]

@phate looks good, but HLS is unhappy

[phate]

@haved I adjusted the golden cycles to appease HLS