[SDA] Bug fix: Use IPD outside the loop as divergence bound

Summary:
The immediate post dominator of the loop header may be part of the divergent loop.
Since this /was/ the divergence propagation bound the SDA would not detect joins of divergent paths outside the loop.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: mmasten, arsenm, jvesely, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59042

llvm-svn: 358681