[InstCombine] add tests for implied simplifications; NFC

Ideally, we would fold all of these in InstSimplify in a
similar way to rL347896, but this is a bit awkward when
we're trying to simplify a compare directly because the
ValueTracking API expects the compare as an input, but
in InstSimplify, we just have the operands of the compare.

Given that we can do transforms besides just simplifications,
we might as well just extend the code in InstCombine (which 
already does simplifications with constant operands).

llvm-svn: 348312