coroutines end and DFG

[Leopold Toetsch]

Does a couroutine end? And how?
The current implementation jumps forth and back by the B<invoke> opcode, 
one is a "call coroutine" the other is a "yield". There seems to be no 
possibility, that the coroutine states, "I'm finished now" - with 
possibly meaning "don't call me again".

And from imcc's register allocations POV the whole coroutine stuff is a 
PITA, no registers are preserved, AFAIK, so a coroutines and the caller 
+ all edges out from both do share the same registers, don't they?

leo