Add IDEA data-flow analysis to guess nullability

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

j2k/testData/multiFile/NullabilityByDFa/SomeServiceUsage.java

@@ -0,0 +1,53 @@
+
+public class SomeServiceUsage {
+
+    public SomeService getService() {
+        return SomeService.getInstanceNotNull();
+    }
+
+    public SomeService getServiceNullable() {
+        return SomeService.getInstanceNullable();
+    }
+
+    // elvis
+    public SomeService getServiceNotNullByDataFlow() {
+        SomeService s = SomeService.getInstanceNullable();
+        return s == null ? SomeService.getInstanceNotNull() : s;
+    }
+
+    // nullable, bang-bang
+    public String aString1() {
+        return getServiceNullable().nullableString();
+    }
+
+    // nullable
+    public String aString2() {
+        return getService().nullableString();
+    }
+
+    // not nullable
+    public String aString3() {
+        return getService().notNullString();
+    }
+
+    // nullable, no bang-bang
+    public String aString4() {
+        return getServiceNotNullByDataFlow().nullableString();
+    }
+
+    // not nullable, no bang-bang
+    public String aString5() {
+        return getServiceNotNullByDataFlow().notNullString();
+    }
+
+    // nullable, safe-call
+    public String aString6() {
+        SomeService s = getServiceNullable();
+        if (s != null) {
+            return s.nullableString();
+        } else {
+            return null;
+        }
+    }
+
+}