Fix default argument method calls

In some cases, calls of methods with default arguments were invoked via the
actual generated method, not via the "...$default" accessor

compiler/testData/codegen/box/defaultArguments/function/augmentedAssignment.kt

@@ -0,0 +1,12 @@
+var result = "Fail"
+
+class A
+
+fun A.plusAssign(a: A, s: String = "OK") {
+    result = s
+}
+
+fun box(): String {
+    A() += A()
+    return result
+}