Reapply [ValueLattice] Fix getCompare() for undef values

Relative to the previous attempt, this also updates the
ValueLattice unit tests.

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Resolve the TODO about incorrect getCompare() behavior. This can
be made more precise (e.g. by materializing the undef value and
performing constant folding on it), but for now just return an
unknown result to fix the correctness issue.

This should be NFC in terms of user-visible behavior, because the
only user of this method (SCCP) was already guarding against
UndefValue results.