[SimplifyCFG] FoldValueComparisonIntoPredecessors(): deal with each predecessor only once

If the predecessor is a switch, and BB is not the default destination,
multiple cases could have the same destination. and it doesn't
make sense to re-process the predecessor, because we won't make any changes,
once is enough.

I'm not sure this can be really tested, other than via the assertion
being added here, which fires without the fix.