C#: Add missing assignment data flow steps

Author: hvitved

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -111,7 +111,7 @@ module LocalFlow {
         or
         // An `=` expression, where the result of the expression is used
         e2 = any(AssignExpr ae |
-            ae.getParent() instanceof Expr and
+            ae.getParent() = any(ControlFlowElement cfe | not cfe instanceof ExprStmt) and
             e1 = ae.getRValue()
           ) and
         scope = e2 and

csharp/ql/test/library-tests/dataflow/local/DataFlowStep.expected

@@ -584,6 +584,8 @@
 | LocalDataFlow.cs:475:44:475:53 | nonTainted | LocalDataFlow.cs:477:15:477:24 | access to parameter nonTainted |
 | LocalDataFlow.cs:480:44:480:44 | x | LocalDataFlow.cs:483:21:483:21 | access to parameter x |
 | LocalDataFlow.cs:480:67:480:68 | os | LocalDataFlow.cs:486:32:486:33 | access to parameter os |
+| LocalDataFlow.cs:483:21:483:21 | access to parameter x | LocalDataFlow.cs:483:16:483:21 | ... = ... |
+| LocalDataFlow.cs:486:32:486:33 | access to parameter os | LocalDataFlow.cs:486:26:486:33 | ... = ... |
 | SSA.cs:5:17:5:17 | SSA entry def(this.S) | SSA.cs:67:9:67:14 | access to field S |
 | SSA.cs:5:17:5:17 | this | SSA.cs:67:9:67:12 | this access |
 | SSA.cs:5:26:5:32 | tainted | SSA.cs:8:24:8:30 | access to parameter tainted |

csharp/ql/test/library-tests/dataflow/local/TaintTrackingStep.expected

@@ -734,6 +734,8 @@
 | LocalDataFlow.cs:480:44:480:44 | x | LocalDataFlow.cs:483:21:483:21 | access to parameter x |
 | LocalDataFlow.cs:480:67:480:68 | os | LocalDataFlow.cs:480:67:480:68 | os |
 | LocalDataFlow.cs:480:67:480:68 | os | LocalDataFlow.cs:486:32:486:33 | access to parameter os |
+| LocalDataFlow.cs:483:21:483:21 | access to parameter x | LocalDataFlow.cs:483:16:483:21 | ... = ... |
+| LocalDataFlow.cs:486:32:486:33 | access to parameter os | LocalDataFlow.cs:486:26:486:33 | ... = ... |
 | SSA.cs:5:17:5:17 | SSA entry def(this.S) | SSA.cs:67:9:67:14 | access to field S |
 | SSA.cs:5:17:5:17 | this | SSA.cs:67:9:67:12 | this access |
 | SSA.cs:5:26:5:32 | tainted | SSA.cs:5:26:5:32 | tainted |