[Dominators] Reapply r306892, r306893, r306893.

This reverts commit r306907 and reapplies the patches in the title.
The patches used to make one of the
CodeGen/ARM/2011-02-07-AntidepClobber.ll test to fail because of a
missing null check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306919 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed