[TBAA] Don't generate invalid TBAA when merging nodes

Summary:
Fix a corner case in `MDNode::getMostGenericTBAA` where we can sometimes
generate invalid TBAA metadata.

Reviewers: chandlerc, hfinkel, mehdi_amini, manmanren

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D26635

llvm-svn: 289403