ARM: simplify EmitAtomicBinary64

ATOMIC_STORE operations always get here as a lowered ATOMIC_SWAP, so there's no
need for any code to handle them specially.

There should be no functionality change so no tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203567 91177308-0d34-0410-b5e6-96231b3b80d8