Rollback constraints if isSameType failed second direction (#20109)

It turns out the following assertion does not hold in the current
definition of `isSameType`
```scala 3
val preConstraint = constraint
val isSame = isSubType(tp1, tp2) && isSubType(tp2, tp1)
isSame.ensuring(_ || constraint == preConstraint)
```

I didn't try to form a minimised snippet where this would cause a
problem. But as an example, the code in
#19955 (comment)
produces invalid constraints which lead to suspicious looking
`<notypes>`s in the subtyping trace.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2304,7 +2304,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         Stats.record("cache same type")
         sames = new util.EqHashMap()
       val res =
-        try isSubType(tp1, tp2) && isSubType(tp2, tp1)
+        try rollbackConstraintsUnless(isSubType(tp1, tp2) && isSubType(tp2, tp1))
         finally
           sameLevel -= 1
           sames = savedSames