[APInt] Remove unnecessary checks of rhsWords==1 with lhsWords==1 from udiv and udivrem. NFC

At this point in the code rhsWords is guaranteed to be non-zero and less than or equal to lhsWords. So if lhsWords is 1, rhsWords must also be 1. urem alread had the check removed so this makes all 3 consistent.

llvm-svn: 302930