[NFC][InstCombine] Tests for ((~x) + y) + 1 -> y - x fold fold (PR42459)

To be noted, this pattern is not unhandled by instcombine per-se,
it is somehow does end up being folded when one runs opt -O3,
but not if it's just -instcombine. Regardless, that fold is
indirect, depends on some other folds, and is thus blind
when there are extra uses.

https://bugs.llvm.org/show_bug.cgi?id=42459
https://rise4fun.com/Alive/EPO0

llvm-svn: 364749