[LegalizeDAG] Expand SADDO/SSUBO using SADDSAT/SSUBSAT (PR37763)

If SADDSAT/SSUBSAT are legal, then we can expand SADDO/SSUBO by performing a ADD/SUB and a SADDO/SSUBO and then compare the results.

I looked at doing this for UADDO/USUBO as well but as we don't have to do as many range comparisons I didn't see any/much benefit.

Differential Revision: https://reviews.llvm.org/D58637

llvm-svn: 354866