Replace ir::BlockIdx::NULL with Option<ir::BlockIdx> #7221

Summary

We are using a hack for representing a null block index by doing:

I think it would be better to remove this hack and using a proper Option<BlockIdx> for representing Some(BlockIdx) or a None it would ensure a proper care for a None/Some cases instead of using an if condition (which is more error prone) like:

if target == BlockIdx::NULL {
  ...
} else {
  ...
}