[InstSimplify] clean up div/rem handling; NFCI
[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.



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@313261 91177308-0d34-0410-b5e6-96231b3b80d8

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