[InstCombine] Add folds for `(X + Y) - (W + Z)`

If `Y` and `Z` are constant then we can simplify to `(X - W) + (Y -
Z)`. If `Y == Z` we can fold to `X - W`.

Note these transform exist outside of InstCombine. The purpose of this
commit is primarily to make it so that folds can generate these
simplifiable patterns without having to worry about creating an inf
loop.