Fix 11877 : static opSlice not possible

Relaxes requirements for opSlice syntax rewrites, details in bugzilla : https://d.puremagic.com/issues/show_bug.cgi?id=11877

```D
struct X
{
    static int opSlice(size_t a, size_t b)
    {
        return 42;
    }
}

static assert ( X[1..2] == 42 );
```