compile.c: struct accessors

* compile.c (rb_method_for_self_aref, rb_method_for_self_aset):
  move from iseq.c to build from node instead of arrays.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e