Completely rewrite domset, idom, and domtree implementation.  Now it is based on the algorithm for directly computing immediate dominators presented in this paper:
Completely rewrite domset, idom, and domtree implementation.  Now it is based
on the algorithm for directly computing immediate dominators presented in this
paper:

  A Fast Algorithm for Finding Dominators in a Flowgraph
  T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.

This _substantially_ speeds up construction of all dominator related information.
Post-dominators to follow.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@10301 91177308-0d34-0410-b5e6-96231b3b80d8

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