[FIR] Properly handle flexible types during creation of DNNT

^KT-50788 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/kt50788.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-50788
+// FILE: Mapper.java
+public class Mapper {
+    public <T> T readValue(Class<T> valueType) {
+        return null;
+    }
+}
+
+// FILE: main.kt
+
+fun <T : CharSequence?> foo(mapper: Mapper, cls: Class<T>?) {
+    val result = <!DEBUG_INFO_EXPRESSION_TYPE("T!!")!>mapper.readValue<T>(cls)!!<!> // The type of result is expected to be T & Any here, but in fact it's just T
+    result.length
+}