Refactor mysterious code in ExpressionCodegen

'super' receiver is now generated in visitSuperExpression()

compiler/testData/codegen/super/multipleSuperTraits.kt

@@ -0,0 +1,13 @@
+trait T1 {
+    fun foo() = "O"
+}
+
+trait T2 {
+    fun foo() = "K"
+}
+
+class A : T1, T2 {
+    override fun foo() = super<T1>.foo() + super<T2>.foo()
+}
+
+fun box() = A().foo()