C++: Add comment as suggested in the PR review for github#14708.

Author: MathiasVP

cpp/ql/test/library-tests/ir/range-analysis/test.cpp

@@ -134,6 +134,12 @@ void test_div(int x) {
 struct X { int n; };
 void read_argument(const X *);
 
+// This test exists purely to ensure that modulus analysis terminates in the
+// presence of inexact phi operands. The LoadInstruction on `while(x->n) { ... }`
+// reads from a PhiInstruction with two input operands: an exact operand defined
+// by the StoreInstruction generated by `x->n--` and an inexact operand coming
+// from the WriteSideEffect generated by `read_argument(x)`. If we don't consider
+// the inexact operand modulus analysis fails to terminate.
 void nonterminating_without_operands_as_ssa(X *x) {
   read_argument(x);
   while (x->n) {