X86: inline all atomic operations up to 128-bits.

The backend *can* cope with all of these now, so Clang should give it the
chance. On CPUs without cmpxchg16b (e.g. the original athlon64) LLVM can reform
the libcalls.

rdar://problem/13496295

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212173 91177308-0d34-0410-b5e6-96231b3b80d8