[Power] Use lwsync for non-seq_cst fences

Summary:
hwsync is only required for seq_cst fences, acquire and release one can use
the cheaper lwsync.

Test Plan: Added some cases to atomics.ll + make check-all

Reviewers: jfb, wschmidt

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5317

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218995 91177308-0d34-0410-b5e6-96231b3b80d8