[FIR] Fold flexible types after substitution if possible

compiler/tests-spec/testData/diagnostics/linked/expressions/conditional-expression/p-6/neg/1.1.fir.kt

@@ -53,7 +53,7 @@ public class JavaClassCase3{
 // TESTCASE NUMBER: 3
 fun case3() {
     val x = JavaClassCase3.id(null) // Nothing!
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?..kotlin.Nothing??")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>x<!>
     val a = if (x) {
         "NOK"
     } else "NOK"