vm.ret in block.tul is undecidable (Halting problem) because of bottomUpTree() in binary-tree.go. This makes it hard to tell during value propagation the boolean value of condition (vm.stack.len != stackSize0 + f.sig.results.len) in funcdecl.tul. Therefore, avoid the condition by putting the return values of a Go function into a list.
Add support for zero-initialized Go return values.