This is yet another step towards solving PR14613 (almost there!):
uaddsat X, Y --> (X >u (X + Y)) ? -1 : X + Y usubsat X, Y --> (X >u Y) ? X - Y : 0
We can't count on a sane vector ISA, so override the default (umin/umax) expansion of unsigned add/sub saturate in cases where we do not have umin/umax.
There may be some small AVX1 opportunities still lurking, but I saw regressions if we allow those transforms wholesale, so stopping here to make sure things look right/better.