[KnownBits] Cleanup some misspelling / logic in {u,s}div

Chronically misspelled 'denominator' as 'denuminator' and a few other
cases.

On the logic side, no longer require `RHS` to be strictly positive in
`sdiv`. This in turn means we need to handle a possible zero `denom`
in the APInt division.

Differential Revision: https://reviews.llvm.org/D150921