[InstCombine] reduce sub-with-overflow ==/!= 0

The basic patterns look like this:
https://alive2.llvm.org/ce/z/MDj9EC

The tests have a use of the overflow value too.
Otherwise, existing folds should reduce already.

This was noted as a missing IR fold in:
926e7312

Hopefully, this makes it easier to implement a backend
fix because we should get the same IR regardless of
whether the source used builtins or inline code.