message: add todo item about datalayout
diff: Alive2 TODO list
  ================
  -Making counterexamples easier to read:
  +Making counterexamples easier to read
  +-------------------------------------
  - don't use undef/poison in CEX unless necessary
  @@ -21,3 +22,9 @@ Making counterexamples easier to read:
  - we'd like a reducer for IR, perhaps a C-Reduce hack if bugpoint
  isn't good enough
  +
  +Usability improvements not specifically related to counterexamples
  +------------------------------------------------------------------
  +
  +- (optionally) make a note in the output when an optimization is
  + correct, but would not be correct under a different datalayout
  