Fix graph creation benchmark: Add a single sink node to the test graph....

Fix graph creation benchmark: Add a single sink node to the test graph. Otherwise a lot of time is spent FixupSourceAndSinkEdges().

Run on ************** (72 X 2993 MHz CPUs); 2018-12-13T14:28:47.680634494-08:00
CPU: Intel Skylake Xeon with HyperThreading (36 cores) dL1:32KB dL2:1024KB dL3:24MB
Benchmark                 Time(ns)        CPU(ns)     Iterations
----------------------------------------------------------------
BM_GraphCreation/10/2        66540          86041           8196
BM_GraphCreation/64/2       136484         161598           4272
BM_GraphCreation/512/2      784516         818832            858
BM_GraphCreation/4k/2      7701701        7744640             89
BM_GraphCreation/32k/2    88439608       88672953              8
BM_GraphCreation/10/4        84779         105101           6644
BM_GraphCreation/64/4       177022         204266           3442
BM_GraphCreation/512/4     1089580        1123798            621
BM_GraphCreation/4k/4     10186857       10253981             68
BM_GraphCreation/32k/4   115389099      115635234              6
BM_GraphCreation/10/8       128583         152827           4611
BM_GraphCreation/64/8       287883         318163           2186
BM_GraphCreation/512/8     1957398        2007351            354
BM_GraphCreation/4k/8     17433017       17517949             40
BM_GraphCreation/32k/8   200025621      200436031              3
BM_GraphCreation/10/16      212706         240579           2904
BM_GraphCreation/64/16      477643         509846           1000
BM_GraphCreation/512/16    3553689        3604331            192
BM_GraphCreation/4k/16    32329683       32445653             21
BM_GraphCreation/32k/16  350925702      351627119              2
PiperOrigin-RevId: 225450999