AArch64: implement copy for paired GPR registers.

When doing 128-bit atomics using CASP we might need to copy a GPRPair to a
different register, but that was unimplemented up to now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353383 91177308-0d34-0410-b5e6-96231b3b80d8