Test precision of MulMayOflo

Previously we would merely ensure that MulMayOflo was correct. That is, that it would correctly identify cases where the product would overflow without caring about false-positives.

Here we test a small number of trivial cases that really don't overflow, ensuring that the predicate doesn't simply claim that everything overflows.

This revealed a subtle bug in the AArch64 implementation in ghc!10970 (closed).