avoid coercing 'this' to a supertype; cleaner handling of stackvalues for 'this' and 'super' (KT-2395)

compiler/testData/codegen/regressions/kt2395.kt

@@ -0,0 +1,11 @@
+import java.util.AbstractList
+
+class MyList(): AbstractList<String>() {
+     public fun getModificationCount(): Int = modCount
+     public override fun get(index: Int): String? = ""
+     public override fun size(): Int = 0
+}
+
+fun box(): String {
+    return if (MyList().getModificationCount() == 0) "OK" else "fail"
+}