Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/nullability/NullableVariableDotAccess.kt

@@ -1,5 +1,4 @@
 fun foo(s: String?, b: Boolean): Int {
     if (s == null) println("null")
-    if (b) return s!!.length
-    return 10
+    return if (b) s!!.length else 10
 }