[SDAG] Vector op legalization for overflow ops

Fixes issue reported by aemerson on D57348. Vector op legalization
support is added for uaddo, usubo, saddo and ssubo (umulo and smulo
were already supported). As usual, by extracting TargetLowering methods
and calling them from vector op legalization.

Vector op legalization doesn't really deal with multiple result nodes,
so I'm explicitly performing a recursive legalization call on the
result value that is not being legalized.

There are some existing test changes because expansion happens
earlier, so we don't get a DAG combiner run in between anymore.

Differential Revision: https://reviews.llvm.org/D61692

llvm-svn: 361166