[SDAG] Address post commit review feedback from f8c63a7f

The major change is falling through to ComputeKnownBits when we don't have an implementation of ComputeNumSignBits due to conservatism over scalable vectors.  Right now, we're mostly conservative in the same cases, but this allows our results to improve when we change ComputeKnownBits without also needing to improve ComputeNumSignBits at the same time.