Cleanup from lexical MIR borrowck removal
Lexical MIR borrowck was removed months ago now, and `EndRegion`s are no longer used for MIRI verification.
* Remove `rustc::mir::StatementKind::EndRegion` and the `-Zemit_end_regions` flag
* Use `RegionVid` instead of `Region` in BorrowSet
* Rewrite drop generation to create fewer goto terminators.
r? @nikomatsakis