`Const::size()` returns int, so change iterators that use it to `auto` instead of `size_t`.
For cases where size is being explicitly cast to `int`, use the wrapper that we already have instead: `Yosys::GetSize()`.
Clean up the code up to Yosys standards. Drop detection of
QL_DSP2_MULTADD in io_regs since those cells can't be inferred with
the current flow anyway.