[Dominators] Move IDoms out of DominatorTreeBase and put them in SNCAInfo

Summary: The temporary IDoms map was used only during DomTree calculation. We can move it to SNCAInfo so that it's no longer a DominatorTreeBase member.

Reviewers: sanjoy, dberlin, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34317

llvm-svn: 306574