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

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
half-formed representation ideas:
- tree branch-choice lists, ala huffman coding
- pointers back to the control flow graph for each exit, tagged with their common "up-tree" nodes (the greatest common of which would be the entry)
- take advantage of the fact that only certain CFG node types can be region entry/exit points and thin out the dominator graph itself!