assertion failure on for loop annotation postcondition

### Version Information

* vyper Version (output of `vyper --version`): cef84e82cd00de7eee6559b3de4f9e8d2e9580b4
### What's your issue about?
```vyper
def foo():
    a: DynArray[DynArray[uint256, 10], 10] = []
    for i: DynArray[uint256, 10] in [k for k in a]:
        for j: uint256 in i:
            pass
```
```
AssertionError
```

fails on: `assert len(pre_parser.for_loop_annotations) == 0`

