Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/nullability/nullableField.kt

@@ -1,4 +1,3 @@
-// ERROR: Type mismatch: inferred type is String? but String was expected
 // ERROR: Type mismatch: inferred type is Int? but Int was expected
 // ERROR: Type inference failed. Please try to specify type arguments explicitly.
 // ERROR: Using 'remove(Int): T' is an error. Use removeAt(index) instead.
@@ -50,7 +49,7 @@ class Test {
 
     }
 
-    fun foo2(s: String) {
+    fun foo2(s: String?) {
 
     }