[InstCombine] add test to show potential nonnull attribute propagation; NFC

This will change with the current draft of:
https://reviews.llvm.org/D28204


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290797 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed