C++: Add testcase that demonstrates a FP caused by spurious flow through phi nodes in IR dataflow.

Author: MathiasVP

cpp/ql/test/library-tests/dataflow/dataflow-tests/dataflow-ir-consistency.expected

@@ -570,6 +570,9 @@ postWithInFlow
 | test.cpp:481:24:481:30 | FieldAddress [post update] | PostUpdateNode should not be the target of local flow. |
 | test.cpp:481:24:481:30 | content [post update] | PostUpdateNode should not be the target of local flow. |
 | test.cpp:482:8:482:16 | VariableAddress [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:489:7:489:7 | VariableAddress [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:491:5:491:5 | x [post update] | PostUpdateNode should not be the target of local flow. |
+| test.cpp:494:5:494:5 | x [post update] | PostUpdateNode should not be the target of local flow. |
 | true_upon_entry.cpp:9:7:9:7 | VariableAddress [post update] | PostUpdateNode should not be the target of local flow. |
 | true_upon_entry.cpp:10:12:10:12 | VariableAddress [post update] | PostUpdateNode should not be the target of local flow. |
 | true_upon_entry.cpp:10:27:10:27 | VariableAddress [post update] | PostUpdateNode should not be the target of local flow. |

cpp/ql/test/library-tests/dataflow/dataflow-tests/test.cpp

@@ -481,4 +481,16 @@ void local_field_flow_def_by_ref_steps_with_local_flow(MyStruct * s) {
   writes_to_content(s->content);
   int* p_content = s->content;
   sink(*p_content);
-}
+}
+
+bool unknown();
+
+void regression_with_phi_flow(int clean1) {
+  int x = 0;
+  while (unknown()) {
+    x = clean1;
+    if (unknown()) { }
+    sink(x); // $ SPURIOUS: ir
+    x = source();
+  }
+}