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

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

@@ -0,0 +1,10 @@
+fun foo(y: String?) {
+    var x: String? = ""
+    if (x != null) {
+        with(y?.let { x = null; it }) {
+            this.length
+            x.length
+        }
+        x.length
+    }
+}