C++: Another small refactor.

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/SsaImpl.qll

@@ -232,8 +232,8 @@ abstract class DefImpl extends TDefImpl {
    */
   abstract predicate isCertain();
 
-  /** Gets the value written to the destination variable by this definition. */
-  abstract Node0Impl getValue();
+  /** Gets the value written to the destination variable by this definition, if any. */
+  Node0Impl getValue() { none() }
 
   /** Gets the operand that represents the address of this definition, if any. */
   Operand getAddressOperand() { none() }
@@ -324,8 +324,6 @@ abstract private class DefAddressImpl extends DefImpl, TDefAddressImpl {
 
   final override predicate isCertain() { any() }
 
-  final override Node0Impl getValue() { none() }
-
   override Cpp::Location getLocation() { result = v.getLocation() }
 
   final override SourceVariable getSourceVariable() {
@@ -653,8 +651,6 @@ class GlobalDefImpl extends DefImpl, TGlobalDefImpl {
 
   override int getIndirection() { result = indirectionIndex }
 
-  override Node0Impl getValue() { none() }
-
   override predicate isCertain() { any() }
 
   /**