pratt parser panics on `unreachable!`

```
contract C {
    int255 public a = -0x//4200 >> 8;
}
// ----
// a() -> -66

```
```
thread '<unnamed>' panicked at 'internal error: entered unreachable code: Pratt parser produced an invalid result', /Users/alpharush/.cargo/git/checkouts/slang-30d565f6b9959ce1/c542db9/crates/solidity/outputs/cargo/crate/src/legacy/generated/parsers.rs:3716:22
```