SI-7517 Fix higher kinded type inference regression

 - Discovered in 2.10.2-RC1
 - Ostensibly regressed in 7e52fb910b, which conceptually reverted
   part of 0cde930b so that (mutable) TypeVars don't use structural equality.
 - But, does *not* fail if 7e52fb910b is cherry-picked directly after 0cde930b,
   suggesting that it shone a light on a behaviour change in some other commit
   in between the two.
 - Indeed, the true regression came in https://github.com/scala/scala/commit/e5da30b843#L5L3176
 - A targeted revert of e5da30b843 is undesirable, as we'd like SI-6846 to stay fixed

What's happening here? In the enclosed test case, higher kinded type
inference explores two possibilities:

    Composed.this.Split[A]
    K[[T]A[B[T]]]           // `Split[A]` dealiased

The difference in the flow of type inference can be seen from the diff
below. Notice how now we no longer register `?K.addBound(Composed.this.Split)`,
we instead only register `?K.addBound(K)`

```patch
--- sandbox/old.log 2013-05-30 00:27:34.000000000 +0200
+++ sandbox/new.log 2013-05-30 00:28:28.000000000 +0200
@@ -1,55 +1,114 @@
 ?K.unifyFull(Composed.this.Split[A])
   ?K.unifySpecific(Composed.this.Split[A])
-  ?K.addBound(Composed.this.Split)
     ?B.unifyFull(T)
       ?B.unifySpecific(T)
       `-> false
     ?B.unifyFull(Any)
       ?B.unifySpecific(Any)
       `-> false
   `-> false
   ?K.unifySpecific(L[[T]A[B[T]]])
-  ?K.addBound(L)
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
+  ?K.addBound(L)
   `-> true
 ?K.unifyFull(Composed.this.Split[A])
   ?K.unifySpecific(Composed.this.Split[A])
-  ?K.addBound(Composed.this.Split)
     ?B.unifyFull(x)
       ?B.unifySpecific(x)
       `-> false
   `-> false
   ?K.unifySpecific(L[[T]A[B[T]]])
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
   ?K.addBound(L)
+  `-> true
+?K.unifyFull(Composed.this.Split[A])
+  ?K.unifySpecific(Composed.this.Split[A])
+    ?B.unifyFull(T)
+      ?B.unifySpecific(T)
+      `-> false
+    ?B.unifyFull(Any)
+      ?B.unifySpecific(Any)
+      `-> false
+  `-> false
+  ?K.unifySpecific(L[[T]A[B[T]]])
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(Composed.this.Split[A])
+  ?K.unifySpecific(Composed.this.Split[A])
+    ?B.unifyFull(x)
+      ?B.unifySpecific(x)
+      `-> false
+  `-> false
+  ?K.unifySpecific(L[[T]A[B[T]]])
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(L[A])
+  ?K.unifySpecific(L[A])
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(L[A])
+  ?K.unifySpecific(L[A])
+  ?K.addBound(L)
   `-> true
```
Merge pull request #2607 from retronym/ticket/7517

SI-7517 Fix higher kinded type inference regression