DAG: Add nuw when splitting loads and stores

DAG: Add nuw when splitting loads and stores

The object can't straddle the address space
wrap around, so I think it's OK to assume any
offsets added to the base object pointer can't
overflow. Similar logic already appears to be
applied in SelectionDAGBuilder when lowering
aggregate returns.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319272 91177308-0d34-0410-b5e6-96231b3b80d8