[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letStable.fir.kt

@@ -0,0 +1,10 @@
+// KT-9051: Allow smart cast for captured variables if they are not modified
+
+fun foo(y: String?) {
+    var x: String? = ""
+    if (x != null) {
+        y?.let { x != y }
+        // x is not changed, smart cast is possible
+        x.length
+    }
+}