Fix failing assertion in ChangeFunctionTypeFix

idea/testData/quickfix/override/typeMismatchOnOverride/afterPropertyReturnTypeMismatchOnOverride.kt

@@ -0,0 +1,8 @@
+// "Change 'x' type to 'Int'" "true"
+trait X {
+    val x: Int
+}
+
+class A : X {
+    override val x: Int = 42
+}

idea/testData/quickfix/override/typeMismatchOnOverride/beforePropertyReturnTypeMismatchOnOverride.kt

@@ -0,0 +1,8 @@
+// "Change 'x' type to 'Int'" "true"
+trait X {
+    val x: Int
+}
+
+class A : X {
+    override val x: Number<caret> = 42
+}