[DA] propagate loop live-out values that get used in a branch

Values that are uniform within a loop but appear divergent to uses
outside the loop are "tainted" so that such uses are marked
divergent. But if such a use is a branch, then it's divergence needs
to be propagated. The simplest way to do that is to put the branch
back in the main worklist so that it is processed appropriately.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D81822