SI-7180 Fix regression in implicit scope of HK type alias.

We actually need to call normalize here, otherwise we don't
progress through #1 below.

    [infer implicit] scala.this.Predef.implicitly[Higher[Foo.Bar]] with pt=Higher[Foo.Bar] in object Foo
    1. tp=Foo.Bar tp.normalize=[A <: <?>]Foo.Bar[A] tp.dealias=Foo.Bar
    2. tp=Foo.Bar[A] tp.normalize=Box[A] tp.dealias=Box[A]

Review by @adriaanm

I'm submitting against 2.10.1 as I think it meets the criteria for an RC3: it's a regression against 2.10.0, its very difficult for users to diagnose, and the workaround is unpalatable (manually expand the type alias at the point of implicit search breaks the users abstractions).