20432c5 introduced a regression where type expressions like the
following would raise a compiler error instead of successfully
compiling:
```
@pure
def f():
    convert(..., uint256)  # raises `not a variable or literal: 'uint256'`
```

the reason is because `get_expr_info` is called on `uint256`, which
is not a regular expr. this commit introduces a fastpath return to
address the issue. longer-term, we should generalize the rule so that
ast traversal does not progress into type expressions.