Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/issues/kt-824-isDir.kt

@@ -11,9 +11,9 @@ object Test {
             return false
         }
         val result = true
-        if (parent.isDirectory) {
-            return true
+        return if (parent.isDirectory) {
+            true
         } else
-            return false
+            false
     }
 }

j2k/testData/fileOrElement/issues/kt-852.kt

@@ -8,9 +8,7 @@ internal class Test {
         if (s1.isEmpty() && s2.isEmpty())
             return "OK"
 
-        if (s1.isEmpty() && s2.isEmpty() && s3.isEmpty())
-            return "OOOK"
+        return if (s1.isEmpty() && s2.isEmpty() && s3.isEmpty()) "OOOK" else ""
 
-        return ""
     }
 }