[PATCH] Added instruction combine to transform few more negative values addition to subtraction

[Dinesh Dwivedi]

Thanks Philip.

If there is any specific part of the patch which is bothering you and you 
are not sure for its correctness. I can explain why I have written it like that

and please don't think about wasting time. I mentioned this before too. I like
to hear from you so please continue review my patches if time permits.

http://reviews.llvm.org/D3733