[InstCombine] Add new combine to add folding

(X | C1) + C2 --> (X | C1) ^ C1 iff (C1 == -C2)

I verified the correctness using Alive:
https://rise4fun.com/Alive/YNV

This transform enables the following transform that already exists in
instcombine:

(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
transform:

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

llvm-svn: 360185