restructured control flow tests

compiler/testData/cfg/expressions/Assignments.kt

@@ -0,0 +1,17 @@
+class Test {
+  var x : Int;
+}
+
+fun assignments() : Unit {
+    var x = 1
+    x = 2
+    x += 2
+
+    x = if (true) 1 else 2
+
+    val y = true && false
+    val z = false && true
+
+    val t = Test();
+    t.x = 1
+}