[cleanup] reduce indentation

Currently, `semantic` function of each *leaf* expressions have two styles:

```cpp
Expression *XXXExp::semantic(Scope *sc)
{
    if (!type) { ... }
}
```

and 

```cpp
Expression *XXXExp::semantic(Scope *sc)
{
    if (type) return this;
    ...
}
```

This pull request arranges code style to the latter, and reduces indentation.