* Introduce helper functions on Module.WipZirCode and zir.Code
 * Move some logic around
 * re-introduce ref_start_index
 * prefer usize for local variables + `@intCast` at the end.
   Empirically this is easier to optimize.
 * Avoid using mem.{bytesAsSlice,sliceAsBytes} because it incurs an
   unnecessary multiplication/division which may cause problems for the
   optimizer.
 * Use a regular enum, not packed, for `Ref`. Memory layout is
   guaranteed for enums which specify their tag type. Packed enums have
   ABI alignment of 1 byte which is too small.