Add missing case to switch statement - DAGTypeLegalizer::ExpandIntegerResult
should expand ATOMIC_CMP_SWAP nodes the same way that it does for ATOMIC_SWAP.

Since ATOMIC_LOADs on some targets (e.g. older ARM variants) get legalized to
ATOMIC_CMP_SWAPs, the missing case had been causing i64 atomic loads to crash
during isel.

<rdar://problem/14074644>