RFC: Single-entry / multiple-exit regions for borrows

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
Or the more general definition, which subsumes the first condition:

> For all vertices V in R, for all vertices U not in R, every path from U to V contains Entry.

Instead of evaluating paths only from the CFG entry to V, evaluating from all nodes not in R allows you to avoid doing any counting.