[InstCombine] foldUnsignedUnderflowCheck(): handle last few cases (PR43251)

Summary:
I don't have a direct motivational case for this,
but it would be good to have this for completeness/symmetry.

This pattern is basically the motivational pattern from
https://bugs.llvm.org/show_bug.cgi?id=43251
but with different predicate that requires that the offset is non-zero.

The completeness bit comes from the fact that a similar pattern (offset != zero)
will be needed for https://bugs.llvm.org/show_bug.cgi?id=43259,
so it'd seem to be good to not overlook very similar patterns..

Proofs: https://rise4fun.com/Alive/21b

Also, there is something odd with `isKnownNonZero()`, if the non-zero
knowledge was specified as an assumption, it didn't pick it up (PR43267)

With this, i see no other missing folds for
https://bugs.llvm.org/show_bug.cgi?id=43251

Reviewers: spatel, nikic, xbolva00

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67412

llvm-svn: 372257