Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/nullability/Overrides.kt

@@ -25,7 +25,7 @@ internal class C : Base(), I {
         return ""
     }
 
-    override fun zoo(o: Any?): String? {
+    override fun zoo(o: Any?): String {
         return ""
     }
 }