DomTree: remove explicit use of DomTreeNodeBase::iterator

Summary:
Almost all uses of these iterators, including implicit ones, really
only need the const variant (as it should be). The only exception is
in NewGVN, which changes the order of dominator tree child nodes.

Change-Id: I4b5bd71e32d71b0c67b03d4927d93fe9413726d4

Reviewers: arsenm, RKSimon, mehdi_amini, courbet, rriddle, aartbik

Subscribers: wdng, Prazek, hiraditya, kuhar, rogfer01, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, vkmr, Kayjukh, jurahul, msifontes, cfe-commits, llvm-commits

Tags: #clang, #mlir, #llvm

Differential Revision: https://reviews.llvm.org/D83087