Method::restore_unshareable_info() does not invoke Method::link_method()

While working on JDK-8169711, I noticed that the assert in Method::link_method() is not always triggered (sometimes we just crash). This is because
1) the "_from_compiled_entry == NULL" check in Method::restore_unshareable_info() is always false and therefore link_method() is not invoked and
2) in Method::link_method() we only execute the check if the adapter (which is shared) was not yet initialized. See attached method.cpp.patch for a suggestion on how to refactor the method.