[InstSimplify] refactor udiv/urem code and add tests; NFCI

This removes some duplicated code and makes it easier to support signed div/rem
in a similar way if we want to do that. Note that the existing comments were not
accurate - we don't need a constant divisor to simplify; icmp simplification does
more than that. But as the added tests show, it could go even further.

llvm-svn: 312885