yield outside loop + break leads to internal error

``` nimrod
iterator foo(x: int): int =
  yield x
  yield x + 1

for i in foo(9):
  if i > 10: break   # internal error: no loop to break
```

```
cgstmts.nim(87)         genSimpleBlock
ccgstmts.nim(828)        genStmts
ccgstmts.nim(307)        genBreakStmt
msgs.nim(669)            InternalError
```
