[InstCombine] add motivational comment for tests; NFC

The referenced tests are derived from:
https://bugs.llvm.org/show_bug.cgi?id=32791
and:
https://reviews.llvm.org/D33172

The motivation for including negative tests may not be clear, so I'm adding an explanatory comment here.
In the post-commit thread for r303133:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170515/453793.html
...it was mentioned that we don't want to add redundant tests. This is a valid point. But in this case, 
we have a patch under review (D33172) that demonstrates that no existing regression tests are affected by
a proposed code change, but these are. Therefore, I think these tests have value not visible in any 
existing regression tests regardless of whether they show a transform.

Differential Revision: https://reviews.llvm.org/D33242

llvm-svn: 303185