Optimize insert operations following find operations

[RRozak]

Often scenario is that insert operation follows find operation if the element isn't found. In this case, looking for the element is performed twice. This can be optimized by using [] operator and getting a reference to the element that was just added if it hasn't existed. I tested it on 10 runs of black_parrot. It saves a few seconds in cts and grt phases. Master:	stage	min time [min]	avg time [min]	med time [min]	max time [min]	sum time [min]
4_1_cts.log	5:04	5:16	5:16	5:22	52:44
5_1_grt.log	11:07	11:18	11:17	11:31	113:03

My changes:	stage	min time [min]	avg time [min]	med time [min]	max time [min]	sum time [min]
4_1_cts.log	5:04	5:11	5:11	5:18	51:50
5_1_grt.log	10:57	11:12	11:16	11:20	112:04

[maliberty]

Issues or PRs should be filed with https://github.com/parallaxsw/OpenSTA if still relevant. This is effectively a fork (though not strictly for historical reasons).

[maliberty]

Issues or PRs should be filed with https://github.com/parallaxsw/OpenSTA if still relevant. This is effectively a fork (though not strictly for historical reasons).