Fix setting bits of parameters in setundef pass

[kamilrakoczy]

What are the reasons/motivation for this change?

This PR fixes setting bits of parameters in setundef pass.

Explain how this is achieved.

Currently we are looping through module parameters using for (auto bit : parameter.second).
It is equivalent to:

for( auto _bit = parameter.second.begin(); _bit != parameter.second.end(); _bit++) {
    auto bit = *_bit;
    ...
}

where auto is resolved to RTLIL::Const.

Iterator that we are using is defined in Const class which iterates through State enum.

operator * of iterator returns copy of the state.

This PR changes to use reference instead.

If applicable, please suggest to reviewers how they can test the change.

I attached example test case that showcase the problem.

Fixes: #4732