Revert "[ObjC][ARC] Check the basic block size before calling DominatorTree::dominate"

This reverts commit 5c3117b0

This should not be necessary after
7593a480, and Florian Hahn has confirmed
that the problem no longer reproduces with this patch.

I happened to notice this code because the FIXME talks about
OrderedBasicBlock.

Reviewed By: fhahn, dexonsmith

Differential Revision: https://reviews.llvm.org/D76075