to introduce additional optimization for another `cc->me->def->type`
later. I carved out the `cc->me->def->type == VM_METHOD_TYPE_ISEQ`
part because I wanted to check other types as well.
mjit_compile.c: drop get_iseq_if_available and define simplified version
of it, has_valid_method_type.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63199 b2dd03c8-39d4-4d8f-98ff-823fe69b080e