This patch changes the definition of negative from -0..-255 to -1..-255. I am changing this because of
a bug that we had in some of the patterns that assumed that "subs" of zero does not set the carry flag.
rdar://12028498
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167963 91177308-0d34-0410-b5e6-96231b3b80d8
