[DAG] Remove (dead) legalization for atomic LoadSDNode and StoreSDNode

This should have been part of 943f3e52 which removed the never completed
migration code which added these.  I left them out because I thought there
was more generic SDAG code to cleanup, but I'd forgotten that SystemZ
relied on custom legalizing ATOMIC_LOAD to (atomic) LoadSDNode.  As a
result, we still need the various legality checks on combines and the
common infrastructure to suport them.