C++: Add 'asIndirectInstruction' as a public predicate.

Author: MathiasVP

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

@@ -321,6 +321,12 @@ module Public {
      */
     Operand asIndirectOperand(int index) { hasOperandAndIndex(this, result, index) }
 
+    /**
+     * Gets the instruction that is indirectly tracked by this node behind
+     * `index` number of indirections.
+     */
+    Instruction asIndirectInstruction(int index) { hasInstructionAndIndex(this, result, index) }
+
     /**
      * Holds if this node is at index `i` in basic block `block`.
      *