[xla:cpu] Optimize async kernel launch for a single task
[xla:cpu] Optimize async kernel launch for a single task

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Benchmark                                        Time             CPU   Iterations
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BM_HostKernelSyncLaunch/1/process_time       11.0 ns         11.0 ns     62432863
BM_HostKernelAsyncLaunch/1/process_time      13.7 ns         13.7 ns     50849533

~2-3 ns lost in loading ready async value from a static variable
