Remove unnecessary work in dominators computation.

Also rename 'undomid' to better indicate how it is being used.

This fixes a performance problem with the dominators computation for
some pathological cases, in one case reducing the time to compute
dominators from 10 minutes down to a few seconds.

Bug: 33957507
Test: m ahat-test
Test: manually verify overview and rooted numbers are unchanged for a
      reasonably complex heap dump.

Change-Id: I2a13f6b62f0bf56e6051da637d9872ea8f8b3d2d