Special case for TypeRefs in upperbound

Done for efficiency. Also that way we would do something
meaningful if the type constructor was, say, Nothing.
However, that case is now rules out anyways.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -2748,7 +2748,12 @@ object Types {
     override def computeHash = doHash(tycon, args)
 
     def upperBound(implicit ctx: Context): Type = tycon match {
-      case tp: TypeProxy => tp.underlying.bounds.hi.appliedTo(args)
+      case tp: TypeRef =>
+        tp.info match {
+          case TypeBounds(_, hi) => hi.appliedTo(args)
+          case _ => tp
+        }
+      case tp: TypeProxy => tp.underlying.appliedTo(args)
       case _ => defn.AnyType
     }