Replace construction of PathVertex objects with updating existing structure in Crpr computation

[glatosinski]

This PR:

• Modifies clkPathPrev so that instead of calling PathVertex constructor to create temporary object it updates the existing object passed by reference
• Updates vertex levels in findCrpr only when necessary.

The results of this modification for Ariane design look as follows (for cts and grt steps, 10 runs):

master cts	PR cts	master grt	PR grt
10:23	11:11	9:45	10:25
11:54	10:57	10:59	10:32
11:44	10:49	11:01	10:52
11:17	11:05	11:18	10:24
11:45	12:36	10:48	12:54
11:35	10:08	10:55	9:37
11:38	10:03	10:44	9:37
11:31	10:03	10:09	9:35
10:25	10:02	9:48	9:39
10:25	10:02	9:50	9:38

This PR could be potentially combined with https://github.com/The-OpenROAD-Project/OpenSTA/pull/211, both affect neighboring parts of code.