[InstCombine] Fold (A - B) u>=/u< A --> B u>/u<= A iff B != 0

https://rise4fun.com/Alive/KtL

This also shows that the fold added in D67412 / r372257
was too specific, and the new fold allows those test cases
to be handled more generically, therefore i delete now-dead code.

This is yet again motivated by
D67122 "[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour"

llvm-svn: 372912