Fixed issue which disallowed using operator[] with TopLevelVariables

[Tiko7454]

bytes16 constant HEX = '0123456789ABCDEF';

contract C {
    function f(uint i) public {
        HEX[1];
    }
}

This code was throwing an assertion error due to HEX was expected to be an lvalue, but it doesn't need to be

[0xalpharush]

Would you mind making this PR against the dev branch please?

[montyly]

Nice catch, thanks @Tiko7454