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.

llvm-svn: 10301