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.kt

@@ -0,0 +1,25 @@
+// TARGET_BACKEND: JVM
+// FILE: Foo.java
+
+public class Foo {
+    public abstract static class Bar extends Foo {
+
+    }
+
+    public static class Buz extends Bar {
+        public String str = "OK";
+    }
+}
+
+// FILE: Box.kt
+
+class Box<out T : Foo>(val foo: T) {
+    val str: String
+        get() {
+            var foo: Foo = foo
+            if (foo is Foo.Buz) {
+                return foo.str
+            }
+            return "";
+        }
+}