[LayoutPass] Add a method to check the followon graph structure.

Found that having a method to check the strucutre of the followon graph makes
it easy to debug file readers. The method checks if there's no wrong edge in
followOnNexts and followOnRoots. It is called only when debuggging is enabled
for LayoutPass.

Reviewers: shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D922

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183553 91177308-0d34-0410-b5e6-96231b3b80d8