pretty-printer omits semicolon needed to disambiguate unary op

```
rustc --pretty normal src/test/run-pass/block-expr-precedence.rs
```

The last line,

```
if (true) { 12 };;; -num;
```

is pretty-printed as

```
if (true) { 12 }
-num;
```

which, when parsed, hits the precedence issue described in the test's comment.
