[Reassociate] Similar to "X + -X" -> "0", added code to handle "X + ~X" -> "-1".

Handle "X + ~X" -> "-1" in the function Value *Reassociate::OptimizeAdd(Instruction *I, SmallVectorImpl<ValueEntry> &Ops);
This patch implements:
TODO: We could handle "X + ~X" -> "-1" if we wanted, since "-X = ~X+1".

Patch by Rahul Jain!

Differential Revision: http://reviews.llvm.org/D3835

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209973 91177308-0d34-0410-b5e6-96231b3b80d8