Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/detectProperties/GetterTypeNotMatch.kt

@@ -1,7 +1,6 @@
-// ERROR: Type mismatch: inferred type is String? but Any was expected
 internal class A {
     private val s: String? = null
 
-    val value: Any
+    val value: Any?
         get() = s
 }