[DomTreeUpdater] Add all insert before all delete updates to reduce compile time.

Summary:
The cleanup in D62751 introduced a compile-time regression due to the way DT updates are performed.
Add all insert edges then all delete edges in DTU to match the previous compile time.
Compile time on the test provided by @mstorsjo before and after this patch on my machine:
113.046s vs 35.649s
Repro: clang -target x86_64-w64-mingw32 -c -O3 glew-preproc.c; on https://martin.st/temp/glew-preproc.c.

Reviewers: kuhar, NutshellySima, mstorsjo

Subscribers: jlebar, mstorsjo, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62981

llvm-svn: 362839