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

compiler/testData/diagnostics/tests/smartCasts/kt3711.fir.kt

@@ -0,0 +1,9 @@
+data class StringPair(val first: String, val second: String)
+
+infix fun String.to(second: String) = StringPair(this, second)
+
+fun f(a: String?) {
+    if (a != null) {
+        val b: StringPair = a to a
+    }
+}