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

[Dinesh Dwivedi]

Hi rafael, bkramer,

This patch enable following transform

  (x + (~(y & c) + 1)   -->   x - (y & c)
  (x + (~((y >> z) & c) + 1)   -->   x - ((y>>z) & c)
 
  (x + (~(y | c) + 1)   -->   x - (y | c) if c is odd
  (x + (~((y >> z) | c) + 1)   -->   x - ((y>>z) | c)  if c is odd

Z3 verification code:
  http://rise4fun.com/Z3/ZA06

http://reviews.llvm.org/D3733

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/add2.ll
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