Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/callChainExpression/libraryMethodCallFromInstance-settings.kt

@@ -3,7 +3,7 @@
 internal class Library {
     fun call() {}
 
-    val string: String?
+    val string: String
         get() = ""
 }
 
@@ -11,9 +11,9 @@ internal class User {
     fun main() {
         val lib: Library = Library()
         lib.call()
-        lib.string!!.isEmpty()
+        lib.string.isEmpty()
 
         Library().call()
-        Library().string!!.isEmpty()
+        Library().string.isEmpty()
     }
 }