64-bit ops are expanded via a cmpxchg8b loop.

64/32-bit and/or/xor are also expanded into a cmpxchg /
cmpxchg8b loop.

Add a cross test for atomic RMW operations and
compare and swap.

Misc: Test that atomic.is.lock.free can be optimized out if result is ignored.

TODO:
* optimize compare and swap with compare+branch further down
instruction stream.

* optimize atomic RMW when the return value is ignored
(adds a locked field to binary ops though).

* We may want to do some actual target-dependent basic
block splitting + expansion (the instructions inserted by
the expansion must reference the pre-colored registers,
etc.). Otherwise, we are currently getting by with modeling
the extended liveness of the variables used in the loops
using fake uses.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3882
R=jfb@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/362463002