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

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

@@ -0,0 +1,6 @@
+// !WITH_NEW_INFERENCE
+fun foo(): Int {
+    var i: Int? = 42
+    i = null
+    return i + 1
+}