Add extra test cases for #7807

Thanks to Lionel.

Author: smarter

tests/pos/i7807.scala

@@ -1,3 +1,16 @@
 object Test with
 
   def flip: (x: 0 | 1) => x.type match { case 0 => 1 case 1 => 0 } = ???
+
+  flip(0): 1
+  flip(1): 0
+
+  flip(if ??? then 0 else 1)
+  val n: 0 | 1 = if ??? then 0 else 1
+  flip(n)
+
+  val m: n.type match { case 0 => 1 case 1 => 0 } = flip(n)
+
+  // The following do not work, see discussion in https://github.com/lampepfl/dotty/pull/7835/files/6e60814e69be5c8d60265d4ce4bc1758863c23d8#r361741296:
+  // flip(m)
+  // flip(flip(n))