[InstCombine] Add new combine to add folding
(X | C1) + C2 --> (X | C1) ^ C1 iff (C1 == -C2)
I verified the correctness using Alive:
This transform enables the following transform that already exists in
(X | Y) ^ Y --> X & ~Y
As a result, the full expected transform is:
(X | C1) + C2 --> X & ~C1 iff (C1 == -C2)
There already exists the transform in the sub case:
(X | Y) - Y --> X & ~Y
However this does not trigger in the case where Y is constant due to an earlier
X - (-C) --> X + C
With this new add fold, both the add and sub constant cases are handled.
Patch by Chris Dawson.
Differential Revision: https://reviews.llvm.org/D61517