[FuzzMutate] Only use undef when explictly asked to (#84959)

Per discussion in https://github.com/SecurityLab-UCD/IRFuzzer/issues/49

,
generating undef during fuzzing seems to be less fruitful. Let's
eliminate undef in favor of poison unless the user explicitly asked for
it.

Signed-off-by: Peter Rong <PeterRong96@gmail.com>