In the simpler version of the link-eval data structure that we use in dominator

computation, the Ancestor field is always set to the Parent, so we can remove
the explicit link entirely and merge the Parent and Ancestor fields. Instead of
checking for whether an ancestor exists for a node or not, we simply check
whether the node has already been processed. This is simpler if Compress is
inlined into Eval, so I did that as well.

This is about a 3% speedup running -domtree on test-suite + SPEC2000 & SPEC2006,
but it also opens up some opportunities for further improvement.

llvm-svn: 124061