We were previously doing it in DAGCombine.
But we also want to do sub %x, C -> add %x, (sub 0, C) for vectors in DAGCombine.
So if we had sub %x, -1, we'll transform it to add %x, 1,
which combineIncDecVector() will immediately transform back into sub %x, -1,
and here we go again...
I've marked this as NFC since not a single test changes,
but since that 'changes' DAGCombine, probably this isn't fully NFC.