[TargetLowering] Simplify expansion of S{ADD,SUB}O

ISD::SADDO uses the suggested sequence described in the section §2.4 of
the RISCV Spec v2.2. ISD::SSUBO uses the dual approach but checking for
(non-zero) positive.

Differential Revision: https://reviews.llvm.org/D47927

llvm-svn: 373187