DAG: Prevent store value forwarding to distinct addrspace load

DAGCombiner replaces (load const_addr1) directly chained with (store
(val, const_addr2)) with val if address space stripped const_addr1 ==
const_addr2. The patch fixes the issue by checking address spaces as
well.  However, it might makes sense to not to chain together side
effects that belong to different address spaces in the first place and
make SelectionDAG::root address space aware.