Add elaborate_expr_param_bit_ and pass param refs for variable selects

For variable parameter selects (indexed and bit) pass the value being
selected as a parameter reference instead of just a numeric constant
so it is easy to get the underlying parameter information.

Move the parameter bit select code into its own routine and cleanup
the remaining general code.

This should not effect the VVP code, but the ivl_expr_parameter(<expr>)
routine can be used to get at the actual parameter information in the
vlog95 target.