nir/algebraic: Improve application of addition property of equality in late algebraic

I wrote these commits a long, long time ago, but I didn't send them out because... first I was playing around with some other changes in the area, then I just plain forgot. I was reminded when I saw !9254. Since this is a superset of one of the commits in that MR, I decided to cleanup the parts that were good and send them out.

The results listed in the commit messages were measured on top of !9108 (merged).