For retcon and retcon.once coroutines we assume that all uses of spills
can be sunk past coro.begin. This simplifies handling of instructions
that escape the address of an alloca.
The current implementation would have issues if the address of the
alloca is escaped before coro.begin. (It also have issues with casts
before the coro.begin and uses of those casts after the coro.begin)
%alloca_addr = alloca ... %escape = ptrtoint %alloca_addr coro.begin store %escape to %alloca_addr