C#: Remove references to getAFirstReadAtNode.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/Assignable.qll

@@ -500,11 +500,7 @@ class AssignableDefinition extends TAssignableDefinition {
    */
   pragma[nomagic]
   AssignableRead getAFirstRead() {
-    exists(ControlFlowNode cfn | cfn = result.getControlFlowNode() |
-      exists(Ssa::ExplicitDefinition def | result = def.getAFirstReadAtNode(cfn) |
-        this = def.getADefinition()
-      )
-    )
+    exists(Ssa::ExplicitDefinition def | result = def.getAFirstRead() | this = def.getADefinition())
   }
 
   /** Gets a textual representation of this assignable definition. */

csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll

@@ -241,7 +241,7 @@ private Ssa::Definition getAnUltimateDefinition(Ssa::Definition def) {
  * exception.
  */
 private predicate defReaches(Ssa::Definition def, ControlFlowNode cfn) {
-  exists(def.getAFirstReadAtNode(cfn))
+  def.getAFirstRead().getControlFlowNode() = cfn
   or
   exists(ControlFlowNode mid | defReaches(def, mid) |
     SsaImpl::adjacentReadPairSameVar(_, mid, cfn) and