[PATCH] Add hooks for emitLeading/TrailingFence in TargetLowering

[JF Bastien]

================
Comment at: include/llvm/Target/TargetLowering.h:962
@@ +961,3 @@
+  ///   AtomicRMW/AtomicCmpXchg/AtomicStore/AtomicLoad.
+  /// RMW and CAS set both IsStore and IsLoad to true.
+  /// Backends without insertFencesForAtomic() should have a no-op here
----------------
I'm not sure if LLVM uses CAS in comments? It would be better to stay consistent can say CmpXchg.

================
Comment at: include/llvm/Target/TargetLowering.h:963
@@ +962,3 @@
+  /// RMW and CAS set both IsStore and IsLoad to true.
+  /// Backends without insertFencesForAtomic() should have a no-op here
+  virtual void emitLeadingFence(IRBuilder<> &Builder, AtomicOrdering Ord,
----------------
Does this mean that the base implementation below can:
```
assert(!getInsertFencesForAtomic());
```
?

The comment should also `s/insertFencesForAtomic/getInsertFencesForAtomic/`

http://reviews.llvm.org/D4959