[SDAG] Recursively legalize both vector mulo results

Split out from D61692 per RKSimon's suggestion. Vector op
legalization will automatically recursively legalize the returned
SDValue, but we need to take care of the other results ourselves.
Otherwise it will end up getting legalized only during op
legalization, by which point it might be too late (though I'm not
aware of any specific cases right now).

There are codegen differences because expansion occurs earlier now
and we don't get a DAGCombiner run in between.

Differential Revision: https://reviews.llvm.org/D61744

llvm-svn: 360470