Inliner heuristic doesn't select forwarders in traits when going through static super accessor

[lrytz]

The inliner heuristic picks forwarder methods for inlining in general. However, this fails for forwarders defined in a trait that are invoked through the static super accessor:

trait T {
  final def foo = bar // forwarder
  @noinline def bar = 42
}

object TT extends T // gets a mixin forwarder foo, calls static T.foo$

class C {
  def baz(t: T) = t.foo // inlined, so we get T.bar
  def biz = TT.foo      // mixin forwarder is inlined, but not the call to the static super accessor, so we get T.foo$
}

This issue was observed in scala/scala#7753.

The heuristic that selects forwarders generally excludes static super accessors, see https://github.com/scala/scala/blob/82336350de4ac3d45369c83d356b8cb0691f9d9b/src/compiler/scala/tools/nsc/backend/jvm/opt/InlinerHeuristics.scala#L243-L248. However, we should inline the static super accessor if the corresponding trait method is a forwarder itself.