[PPC] Fix atomics lowering in DAG lowering.

I forgot to forward the chain, causing some missing instruction
dependencies. The test crashes the compiler without this patch.

Inspired by the test case, D33519 also tries to remove the extra sync.

Differential Revision: https://reviews.llvm.org/D33573

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303931 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed