fix 'operator' modifier inspection

#KT-9349 Fixed

idea/testData/quickfix/migration/operatorModifier/overridden.kt

@@ -0,0 +1,10 @@
+// "Add 'operator' modifier" "false"
+// ACTION: Convert to block body
+// ACTION: Specify return type explicitly
+open class A {
+    open operator fun plus(a: A) = A()
+}
+
+class B : A() {
+    override fun plu<caret>s(a: A) = A()
+}