As we have discussed previously (e.g. in D63992 / D64090 / PR42457), sub instruction
can almost be considered non-canonical. While we do convert sub %x, C -> add %x, -C,
we sparsely do that for non-constants. But we should.
Here, i propose to interpret sub %x, %y as add (sub 0, %y), %x IFF the negation can be sinked into the %y
This has some potential to cause endless combine loops (either around PHI's, or if there are some opposite transforms).
For former there's -instcombine-negator-max-depth option to mitigate it, should this expose any such issues
For latter, if there are still any such opposing folds, we'd need to remove the colliding fold.
In any case, reproducers welcomed!