[InstCombine] Remove some uninteresting FIXMEs (NFC)

If there are two undef operands, the select would get folded away
entirely. One undef operand can occur if the other two operands
do not satisfy the poison implication check. However, I don't think
that handling this edge case is worthwhile in this fold. If we
wanted to handle this, it would be more natural to do so in the
simplifyValueKnownNonZero() fold (as this is actually the property
we would be exploiting -- this doesn't really have any relation
to taking the log2).