[InstSimplify] clean up div/rem handling; NFCI

The idea to make an 'isDivZero' helper was suggested for the signed case in D37713:
https://reviews.llvm.org/D37713

This clean-up makes it clear that D37713 is just filling the gap for signed div/rem,
removes unnecessary code, and allows us to remove a bit of duplicated code from the
planned improvement in D37713.

llvm-svn: 313261