Revert "Temporarily remove isInitialized and tests, but keep the implementation"

This reverts commit 234148518e.

compiler/testData/codegen/box/properties/lateinit/isInitializedAndDeinitialize/sideEffects.kt

@@ -0,0 +1,21 @@
+// TARGET_BACKEND: JVM
+// LANGUAGE_VERSION: 1.2
+// WITH_RUNTIME
+
+class Foo {
+    lateinit var bar: String
+
+    fun test(): String {
+        var state = 0
+        if (run { state++; this }::bar.isInitialized) return "Fail 1"
+
+        bar = "A"
+        if (!run { state++; this }::bar.isInitialized) return "Fail 3"
+
+        return if (state == 2) "OK" else "Fail: state=$state"
+    }
+}
+
+fun box(): String {
+    return Foo().test()
+}