[InstCombine] add tests for notnotsub; NFC

As shown in D44043, we may need this fold in the backend,
but it's also missing in the IR optimizer.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326659 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed