[InstCombine] add explanatory comment to test; NFC

The test was added at r290797, and a patch to enable the transform is proposed in D28204.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290798 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed