Add IDEA data-flow analysis to guess nullability

Add "if return..." folding to "return if"

j2k/testData/multiFile/NullabilityByDFa/external/JavaFile.java.expected

@@ -0,0 +1,26 @@
+
+public class SomeService {
+
+    public static SomeService getInstanceNotNull() {
+        return new SomeService();
+    }
+
+    public static SomeService getInstanceNullable() {
+        if (Math.random() > 0.5)
+            return null;
+        return new SomeService();
+    }
+
+    public String nullableString() {
+        if (Math.random() < 0.5)
+            return null;
+        return Math.random() + "";
+    }
+
+    public String notNullString() {
+        String s = nullableString();
+        if (s != null)
+            return s;
+        return "null";
+    }
+}