GlobalISel: Implement lower for G_SADDO/G_SSUBO

Port directly from SelectionDAG, minus the path using
ISD::SADDSAT/ISD::SSUBSAT.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375042 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed