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

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
is shrinking/extending part of the lifetime analysis implementation in rustc? if it was instead implemented to gather entry/exit points then solve them all at once, would this make the trivial extension (defining exits via the CFG) feasible?