Add a small "usage:" comment at the top of not.cpp

Mostly pulled from Rafael's r185678 commit message.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222855 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Added inst combine transforms for single bit tests from Chris's note"

This reverts commit r210006, it miscompiled libapr which is used in who
knows how many projects.

A test has been added to ensure that we don't regress again.

I'll work on a rewrite of what the optimization was trying to do later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222856 91177308-0d34-0410-b5e6-96231b3b80d8