Fix trellis layout to avoid mis-identify triangle.

Summary:
For the following CFG:

A->B
B->C
A->C

If there is another edge B->D, then ABC should not be considered as triangle.

Reviewers: davidxl, iteratee

Reviewed By: iteratee

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D31310

llvm-svn: 298661