Fix for KT-3969: Codegen fails on unary calls with default arguments

#KT-3969 Fixed

compiler/testData/codegen/box/extensionFunctions/kt3969.kt

@@ -0,0 +1,19 @@
+var result = "Fail"
+
+class A
+
+fun A.inc(s: String = "OK"): A {
+    result = s
+    return this
+}
+
+fun box(): String {
+    var a = A()
+    a++
+    if (result != "OK") return "Fail 1"
+
+    result = "Fail"
+    ++a
+
+    return result
+}