[PATCH] [Power] Use AtomicExpandPass for fence insertion, and use lwsync where appropriate

[Robin Morisset]

Hi jfb,

This patch makes use of AtomicExpandPass in Power for inserting fences around
atomic as part of an effort to remove fence insertion from SelectionDAGBuilder.
As a big bonus, it lets us use sync 1 (lightweight sync, often used by the mnemonic
lwsync) instead of sync 0 (heavyweight sync) in many cases.

I also added a test, as there was no test for the barriers emitted by the Power
backend for atomic loads and stores.

http://reviews.llvm.org/D5180

Files:
  include/llvm/IR/IntrinsicsPowerPC.td
  lib/Target/PowerPC/PPCISelLowering.cpp
  lib/Target/PowerPC/PPCISelLowering.h
  lib/Target/PowerPC/PPCInstrInfo.td
  lib/Target/PowerPC/PPCTargetMachine.cpp
  test/CodeGen/PowerPC/atomics.ll
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