Fix Issues with cmpxchg example

[Daniel Robertson via llvm-commits]

Reading through the documentation and found that the example for
cmpxchg didn't work. The attached patch addresses that and was generated
with `git format-patch --no-attach master..localbranch`. I appologize
if this is the wrong mailing list. The documentation here
(http://llvm.org/docs/DeveloperPolicy.html) stated that one-off patches
could be sent here.

Cheers,

Daniel
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