[AArch64] Add preliminary support for ARMv8.1 SUB/AND atomics

This patch is a follow-up to r305893 and adds preliminary support for the
fetch_sub and fetch_and operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307913 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed