FIR. Refactor smart-cast representation in FIR tree

Make smart-casts non-transparent expression without delegation
to underlying FirQualifiedAccessExpression, as children delegation in
fir tree has unclear semantics
Remove two different kinds of tree nodes for smart-casts

compiler/testData/ir/irText/firProblems/OutBox.fir.kt.txt

@@ -0,0 +1,21 @@
+class Box<out T : Foo> {
+  constructor(foo: T) /* primary */ {
+    super/*Any*/()
+    /* <init>() */
+
+  }
+
+  val foo: T
+    field = foo
+    get
+
+  val str: String
+    get(): String {
+      var foo: Foo = <this>.<get-foo>()
+      when {
+        foo is Buz -> return foo /*as Buz */(super<Buz>).#str /*!! String */
+      }
+      return ""
+    }
+
+}