Revert "Fix PR 24415 (at least), by making our post-dominator tree behavior sane."

and also "clang-format GenericDomTreeConstruction.h, since the current
formatting makes it look like their is a bug in the loop indentation, and there
is not"

This reverts commit r296535.

There are still some open design questions which I would like to discuss. I
revert this for Daniel (who gave the OK), as he is on vacation.

llvm-svn: 296812