Allow scalable vectors in ComputeNumSignBits and isKnownNonNull

This is a follow up to D136470 which extends the same scheme used there to ComputeNumSignBits and isKnownNonNull. As a reminder, for scalable vectors we track a single bit which is implicitly broadcast to all lanes. We do not know how many lanes there are statically, and thus have to be conservative along paths which require exact sizes.

Differential Revision: https://reviews.llvm.org/D137046