getValueFromCondition() uses a Visited set to record the intermediate value.
However, it uses a postorder way to compute the value first and update the
Visited set later. Thus it will be trapped into an infinite recursion if there
exists IRs that use no dominated by its def as in this example:
"%tmp3 = or i1 undef, %tmp4" "%tmp4 = or i1 undef, %tmp3"
To prevent this, we can insert an Overdefined placeholder into the set
before computing the actual value.