[Dominators] Move number to node mapping out of DominatorTreeBase

Summary: Number to node mapping in DominatorTreeBase is used only during calculation, so there is no point keeping is as a member variable. This patch moves this mapping to Calculate function and passes it to helper functions. It also makes the name more descriptive.

Reviewers: sanjoy, dberlin, davide, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34295

llvm-svn: 306562