The order of iseq may differ from the order of tokens, typically
`while`/`until` conditions are put after the body.

These orders can match by using line numbers as builtin-indexes, but
at the same time, it introduces the restriction that multiple `cexpr!`
and `cstmt!` cannot appear in the same line.

Another possible idea is to use `RubyVM::AbstractSyntaxTree` and
`node_id` instead of ripper, with making BASERUBY 3.1 or later.