Ignore indirect branches from callbr.

Summary:
We can't speculate around indirect branches: indirectbr and invoke. The
callbr instruction needs to be included here.

Reviewers: nickdesaulniers, manojgupta, chandlerc

Reviewed By: chandlerc

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66200

llvm-svn: 368873