This falls naturally out of making drop elaboration work with `box`
expressions, which is probably required for sane MIR borrow-checking.
This is a pure refactoring with no intentional functional effects.

The unused blocks are removed by SimplifyCfg, but they can cause a
significant performance slowdown before they are removed.

The tests use `-Z identify_regions` so one can eyeball output for
sanity. The tests with closures use `-Z span_free_formats` so that
host-specific paths do not get embedded into the dumped MIR.

The tests check against MIR dump output immediately prior to borrowck
(determined by hand to be the dump from after the "qualify-consts"
pass) since that is when `EndRegion` will be most relevant in the near
term.