Eliminate TypeVars and Wildcards early in glb/lub

Type variables and wildcards can act both as subtypes and
supertypes depending on the direction of the test. This means
types with variables and/or wildcards can shadow other types in
`glb/lub`.

Consider the following examples:
```scala
lub((Int, ? ) :: (Int, String) :: Nil)
glb((Int, ?A) :: (Int, String) :: Nil)
```

Intuitively both should return `(Int, String)`, but this depends
on the order of types in the list passed to `glb/lub`.
This commit incorporates the simple heuristic of moving types that
contain variables and/or wildcards to the end of the list.

Fixes scala/bug#10686 and fixes scala/bug#10740

Author: joroKr21

src/reflect/scala/reflect/internal/tpe/GlbLubs.scala

@@ -167,7 +167,14 @@ private[internal] trait GlbLubs {
       case Nil => Nil
     }
 
-    loop(ts)
+    // The order here matters because type variables and
+    // wildcards can act both as subtypes and supertypes.
+    val (ts2, ts1) = ts.partition(_ exists {
+      case tv: TypeVar => !tv.isGround
+      case t => t.isWildcard
+    })
+
+    loop(ts1 ::: ts2)
   }
 
   /** Eliminate from list of types all elements which are a supertype

test/files/pos/t10686.scala

@@ -0,0 +1,6 @@
+object Test {
+  def ltr[A](implicit ev: (Int, A) =:= (Int, String)) = null
+  def rtl[A](implicit ev: (Int, String) =:= (Int, A)) = null
+  ltr
+  rtl
+}

test/files/pos/t10740.scala

@@ -0,0 +1,11 @@
+object Test {
+  case class Inv[A](x: A)
+  def diff1[A](i: Inv[A], j: Inv[_ <: A]) = 1
+  def diff2[A](i: Inv[_ >: A], j: Inv[A]) = 2
+  def diff3[A](i: Inv[_ >: A], j: Inv[_ <: A]) = 3
+  val i = Inv(Option(42))
+  val j = Inv(Some(42))
+  diff1(i, j)
+  diff2(i, j)
+  diff3(i, j)
+}