handler labels

[Matthew Fluet]

I've been working on getting better translations of transfer statements,
hopefully in order to allow some branches to fall through as opposed to
being explicit jumps.  But, I was starting with some simple goto
eliminations -- first eliminate all blocks which are nothing but jump by
replacing jumps to that block with jumps to the final target.  That was
pretty easy.  Then I wanted to combine block A with block B when the only
transfer to block B is jump from block A.  This is having some problems.

Question 1: from the Chunk.t datatype, I get two lists of labels: 
gcReturns and entries.  I understand that gcReturns are the set of labels
that got inserted for limitChecks and prims that enter the runtime.  And
entries are the function entry points.  I had thought that those where the
only ways of "entering" the Chunk.  But handlers are handled differently?
i.e., is it possible to transfer between chunks because of a handle?  I
think that might explain why I'm getting unresolved symbols.  I wanted to
be aggressive and simply count all the jumps to each label and then toss
out anything that has 0 jumps to it.  Obviously, things like gcReturns
have no explicit jumps, so they are marked as Must (be kept).  I guess
handlers are the same way -- some of them are never explicitly jumped to,
yet they must be kept.

I suspect this issue arose in the c-backend as well.  (I think) it
explains why I didn't understand why all the targets of Switches and
SwitchIPs were forced to be output.

Question 2: Any reason not to build up a list of handler labels and pass
them along with the Chunks in order to do this precise analysis?  Or are
there other cases where it would break?