[InstCombine] Fold icmp ugt/ult (add nuw X, C2), C --> icmp ugt/ult X, (C - C2)

Support for sgt/slt was added in rL294898, this adds the same cases also for unsigned compares.

This is the Alive proof: https://rise4fun.com/Alive/nyY

Differential Revision: https://reviews.llvm.org/D50972

llvm-svn: 341353