C++: isConstant() -> exists(getValue())

This code was clearly using `isConstant` as an indirect way of checking
whether `getValue` would have a result. That's no longer valid, so I
changed it to check `getValue` directly.

Author: jbj

cpp/ql/src/Likely Bugs/Likely Typos/LogicalExprCouldBeSimplified.ql

@@ -43,7 +43,7 @@ string comparisonOnLiterals(ComparisonOperation op) {
   simple(op.getLeftOperand()) and
   simple(op.getRightOperand()) and
   not op.getAnOperand().isInMacroExpansion() and
-  if op.isConstant()
+  if exists(op.getValue())
   then result = "This comparison involves two literals and is always " + op.getValue() + "."
   else result = "This comparison involves two literals and should be simplified."
 }