Store refactoring

#KT-1213 Fixed

compiler/testData/codegen/boxWithStdlib/lazyCodegen/safeAssign.kt

@@ -0,0 +1,10 @@
+class Shape(var result: String) {
+
+}
+
+fun box(): String {
+    var a : Shape? = Shape("fail");
+    a?.result = "OK";
+
+    return a!!.result
+}