C#: Remove the no-match steps (they occur due to control flow splitting).

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowGraph.qll

@@ -225,6 +225,13 @@ module ControlFlow {
       result = this.getASuccessorByType(any(BooleanSuccessor t | t.getValue() = false))
     }
 
+    /**
+     * Gets an immediate `match` predecessor, if any.
+     */
+    Node getAMatchPredecessor() {
+      result = this.getAPredecessorByType(any(MatchingSuccessor t | t.isMatch()))
+    }
+
     /** Gets the enclosing callable of this control flow node. */
     final Callable getEnclosingCallable() { result = Impl::getNodeCfgScope(this) }
   }

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

@@ -2597,12 +2597,19 @@ predicate readStep(Node node1, ContentSet c, Node node2) {
     or
     dynamicPropertyRead(node1.asExpr(), c, node2.asExpr())
     or
-    patternPropertyRead(node1.asExpr(), c, node2.asExpr())
-    or
     node2.asExpr().(AwaitExpr).getExpr() = node1.asExpr() and
     c = getResultContent()
   )
   or
+  exists(ReadStepConfiguration x, ControlFlow::Node cfn1, ControlFlow::Node cfn2 |
+    cfn1 = node1.getControlFlowNode() and
+    cfn2 = node2.getControlFlowNode() and
+    cfn1.getAMatchPredecessor() = cfn2 and
+    x.hasExprPath(_, cfn1, _, cfn2)
+  |
+    patternPropertyRead(node1.asExpr(), c, node2.asExpr())
+  )
+  or
   FlowSummaryImpl::Private::Steps::summaryReadStep(node1.(FlowSummaryNode).getSummaryNode(), c,
     node2.(FlowSummaryNode).getSummaryNode())
   or