DomTree: add private create{Child,Node} helpers

Summary:
Aside from unifying the code a bit, this change smooths the
transition to use of future "opaque generic block references"
in the type-erased dominator tree base class.

Change-Id: If924b092cc8561c4b6a7450fe79bc96df0e12472

Reviewers: arsenm, RKSimon, mehdi_amini, courbet

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83086