As opposed to using weirdness involving pretending the body block
is the loop block. This does not pass tests
This commit is [ci skip] because I know it doesn't pass tests yet.
Somehow this commit introduces nondeterminism into the handling of
loops.
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.
