[PATCH] Add AtomicExpandPass::bracketInstWithFences, and use it whenever getInsertFencesForAtomic would trigger in SelectionDAGBuilder

[Reid Kleckner]

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Comment at: lib/Target/ARM/ARMISelLowering.cpp:11052
@@ -11052,2 +11051,3 @@
   }
+  llvm_unreachable("Unknown fence ordering in emitLeadingFence");
 }
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jfb wrote:
> Fold this into the `default` case for the `switch`.
We can't do this because MSVC will warn about falling off the end of the function.

http://reviews.llvm.org/D5179