Liveness analysis for generators assumes that execution always continues
normally after a yield point, not accounting for the fact that generator
could be dropped before completion.

If generators captures any variables by reference, those variables could
be used within a generator, or when the generator completes, but also
after each yield point in the case the generator is dropped.

Account for the case when generator is dropped after yielding, but
before running to the completion. This effectively considers all
variables captured by reference to be used after a yield point.

The changes to dumping expressions seem particularly useful

Otherwise, we can get into a situation where you have
a subtype obligation `#1 <: #2` pending, #1 is constrained
by `check_casts`, but #2` is unaffected.
Co-authored-by: NNiko Matsakis <niko@alum.mit.edu>