[PowerPC] Fix eq/ne comparison of v2i64 pre-Power8

In commit 1674d9b6, I fixed the bug where we didn't consider
both words of the result of the comparison. However, the logic
needs to be different for eq and ne.
Namely for eq, we need both words of the doubleword to equal so it
is an AND. OTOH for ne, we need either word to be unequal so it
is an OR.