[SimplifyCFG] FoldValueComparisonIntoPredecessors(): drop reachable errneous assert

I have added it in d15d81ce because it *seemed* correct, was holding
for all the tests so far, and was validating the fix added in the same
commit, but as David Major is pointing out (with a reproducer),
the assertion isn't really correct after all. So remove it.

Note that the d15d81ce still fine.