[InstSimplify] add/move tests for add folds; NFC

isKnownNegation() is currently proposed as part of D48754,
but it could be used to make InstSimplify stronger independently
of any abs() improvements.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336822 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed