C#: Address review comment.

Author: aschackmull

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

@@ -1291,10 +1291,10 @@ class SsaNode extends NodeImpl, TSsaNode {
 class SsaDefinitionNode extends SsaNode {
   override SsaImpl::DataFlowIntegration::SsaDefinitionNode node;
 
-  SsaImpl::Definition getDefinition() { result = node.getDefinition() }
+  Ssa::Definition getDefinition() { result = node.getDefinition() }
 
   override ControlFlow::Node getControlFlowNodeImpl() {
-    result = this.getDefinition().(Ssa::Definition).getControlFlowNode()
+    result = this.getDefinition().getControlFlowNode()
   }
 }