Noise reduction + minor enhance in TreeCheckers.

Misc irrelevant work, which I can only offer as-is. It lowers
the noise in -Ycheck:* output and performs some common sense
chillaxes like not screaming ERROR IN INTERNAL CHECKING! WE'RE
ALL GOING TO DIE! when a tree doesn't hit all nine points at
the Jiffy Tree.

You can see some reasonably well reduced symbol flailing if
you run the included pending tests:

  test/partest --show-diff test/pending/pos/treecheckers

Example output,

Out of scope symbol reference {
      tree  TypeTree             Factory[Traversable]
  position  OffsetPosition       test/pending/pos/treecheckers/c5.scala:3
  with sym  ClassSymbol          Factory: Factory[CC]
   and tpe  ClassArgsTypeRef     Factory[Traversable]
   encl(1)  ModuleSymbol         object Test5
    ref to  AbstractTypeSymbol   X (<deferred> <param>)
}

1 files changed, 5 insertions, 4 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index 9c66dc476f..ca01a4b8e3 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -4036,12 +4036,13 @@ trait Types
 
   def isConstantType(tp: Type) = tp match {
     case ConstantType(_) => true
-    case _ => false
+    case _               => false
   }
 
-  def isExistentialType(tp: Type): Boolean = tp.dealias match {
-    case ExistentialType(_, _) => true
-    case _                     => false
+  def isExistentialType(tp: Type): Boolean = tp match {
+    case _: ExistentialType           => true
+    case tp: Type if tp.dealias ne tp => isExistentialType(tp.dealias)
+    case _                            => false
   }
 
   def isImplicitMethodType(tp: Type) = tp match {