ZJIT: Add for_each_operand methods to Insn (ruby#16305)

- Add `for_each_operand`, `for_each_operand_mut`, and `try_for_each_operand` methods to `Insn`, backed by a shared `for_each_operand_impl!` macro
- Replace the old `worklist_traverse_single_insn` function on `Function` with direct use of the new methods
- Avoid materializing resolved `Insn` objects at call sites; instead apply union-find inside the callback

Author: tekknolagi