X86: expand atomics in IR instead of as MachineInstrs.

The logic for expanding atomics that aren't natively supported in
terms of cmpxchg loops is much simpler to express at the IR level. It
also allows the normal optimisations and CodeGen improvements to help
out with atomics, instead of using a limited set of possible
instructions..

rdar://problem/13496295

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212119 91177308-0d34-0410-b5e6-96231b3b80d8
AArch64: fix comment typo

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212120 91177308-0d34-0410-b5e6-96231b3b80d8