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

compiler/testData/diagnostics/tests/smartCasts/varnotnull/infix.fir.kt

@@ -0,0 +1,11 @@
+// See KT-774
+fun box() : Int {
+    var a : Int? = 1
+    var d = 1
+
+    if (a == null) {
+        return 2
+    } else {
+        return a + d
+    }
+}