Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/nullability/autoNotNull.kt

@@ -1,8 +1,5 @@
 internal class A {
     fun foo(s: String?): Int {
-        if (s != null) {
-            return s.length
-        }
-        return -1
+        return s?.length ?: -1
     }
 }