backing field migration fixes

idea/testData/inspections/cleanup/cleanup.kt

@@ -35,3 +35,9 @@ fun unnecessaryCast(x: String) = x as String
 fun unnecessaryElvis(x: String) = x ?: ""
 
 @JavaAnn(1, "abc") class MyClass
+
+class Foo {
+    var x: Int = 0
+        get = $x
+        set(value) { $x = value }
+}

idea/testData/inspections/cleanup/cleanup.kt.after

@@ -34,3 +34,9 @@ fun unnecessaryCast(x: String) = x
 fun unnecessaryElvis(x: String) = x
 
 @JavaAnn(1, arg1 = "abc") class MyClass
+
+class Foo {
+    var x: Int = 0
+        get = field
+        set(value) { field = value }
+}