[IPSCCP,PM] Preserve DT in the new pass manager.

After D45330, Dominators are required for IPSCCP and can be preserved.

This patch preserves DominatorTreeAnalysis in the new pass manager. AFAIK the legacy pass manager cannot preserve function analysis required by a module analysis.

Reviewers: davide, dberlin, chandlerc, efriedma, kuhar, NutshellySima

Reviewed By: chandlerc, kuhar, NutshellySima

Differential Revision: https://reviews.llvm.org/D47259

llvm-svn: 346486