[async] Reduce the inlined edges size from 16 to 8

[k-ye]

Related issue = #742

On my end:

16:

...
[I 12/20/20 13:16:02.093] [state_flow_graph.cpp:taichi::lang::StateFlowGraph::benchmark_rebuild_graph@1504] nodes = 1001 total time 866.5100 ms (averaged so far 821.9 ms); per_node 865.6444 ns
[I 12/20/20 13:16:02.184] [state_flow_graph.cpp:taichi::lang::StateFlowGraph::benchmark_rebuild_graph@1504] nodes = 1001 total time 901.1000 ms (averaged so far 822.7 ms); per_node 900.1998 ns

8:

...
[I 12/20/20 13:16:40.974] [state_flow_graph.cpp:taichi::lang::StateFlowGraph::benchmark_rebuild_graph@1504] nodes = 1001 total time 710.3000 ms (averaged so far 721.0 ms); per_node 709.5904 ns
[I 12/20/20 13:16:41.044] [state_flow_graph.cpp:taichi::lang::StateFlowGraph::benchmark_rebuild_graph@1504] nodes = 1001 total time 701.7100 ms (averaged so far 720.8 ms); per_node 701.0090 ns

Note that averaged so far went down from 821 ms to 721 ms, so 12% faster..

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[xumingkuan]

Cool!

[yuanming-hu]

Cool! On my end, it goes down from 601 ns to 572 ns (5% faster) :-)