[Dominators] Move InfoRec outside of DominatorTreeBase

Summary:
The InfoRec struct is used only during tree construction, so there is no point having it as a DominatorTreeBase member.

This patch moves it into the Calculate function instead and makes it pass it to its helper functions.

Reviewers: sanjoy, dberlin, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34305

llvm-svn: 306572