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

compiler/testData/diagnostics/tests/dataFlow/WhenSubject.fir.kt

@@ -0,0 +1,10 @@
+interface Expr
+class BinOp(val operator : String) : Expr
+
+fun test(e : Expr) {
+    if (e is BinOp) {
+        when (e.operator) {
+            else -> 0
+        }
+    }
+}